This will allow substitution of boundaries in mail messages.

Some calls in different modules were setting the data handle to NULL, causing
segmentation faults when using builds that enable character code conversions.

Automake gets confused if you want to use C++ static libraries with C
code - basically we need to involve the clang++ linker. The easiest way
of achieving this is to rename the C code as C++ code. This gets us a
bit further along the path and ought to be compatible with Google's
version of clang.

Closes #1849

Create simple seed corpora for:
- FTP
- telnet
- dict
- tftp
- imap
- pop3

based off the tests of the same number.

Closes #1842

- Start with the basic code from the ossfuzz project.
- Rewrite fuzz corpora to be binary files full of Type-Length-Value
  data, and write a glue layer in the fuzzing function to convert
  corpora into CURL options.
- Have supporting functions to generate corpora from existing tests
- Integrate with Makefile.am

... so that users can actually write code based on the man page alone,
not having to read the header file.

- Fix handling certificate subjects that are already UTF-8 encoded.

Follow-up to b3b75d17 from two days ago. Since then a copy would be
skipped if the subject was already UTF-8, possibly resulting in a NULL
deref later on.

Ref: https://github.com/curl/curl/issues/1823
Ref: https://github.com/curl/curl/pull/1831

Closes https://github.com/curl/curl/pull/1836

... instead of cyassl, as this is the current name for it.

Closes #1844

Reported-by: Dan Fandrich
Bug: https://curl.haxx.se/mail/lib-2017-08/0121.html
Closes #1843

... when darwinssl is used.

Reported-by: Viktor Szakats
Bug: https://github.com/curl/curl/commit/b0989cd3abaff4f9a0717b4875022fa79e33b481#commitcomment-23943493

Closes #1845

./sslbackend.c:58:3: warning: else after closing brace on same line (BRACEELSE)
   } else if(isdigit(*name)) {
   ^
./sslbackend.c:62:3: warning: else after closing brace on same line (BRACEELSE)
   } else
   ^

Closes https://github.com/curl/curl/pull/1840

The CURLSSLBACKEND_WOLFSSL is supposed to be an alias for
CURLSSLBACKEND_CYASSL, but used an erronous value. To reduce the risk
for a similar mistake, define the backend aliases to use the enum values
instead.

Reported-by: Gisle Vanem
Bug: https://curl.haxx.se/mail/lib-2017-08/0120.html

it is a one time *set*, not necessarily a one time use... it can be
called again if the first call failed or just listed the alternatives.

clarify that the available backends are the ones this build supports

plus add some formatting

Reported-by: Rich Gray
Bug: https://curl.haxx.se/mail/lib-2017-08/0119.html

Regression since 1328f69d

Fixes #1841
Reported-by: Andrei Karas

Closes #1837

destroy_async_data() assumes that if the flag "done" is not set yet, the
thread itself will clean up once the request is complete.  But if an
error (generally OOM) occurs before the thread even has a chance to
start, it will never get a chance to clean up and memory will be leaked.
By clearing "done" only just before starting the thread, the correct
cleanup sequence will happen in all cases.

This used to be set in some configurations to EAI_MEMORY which is not a
valid value for errno and caused Curl_strerror to fail an assertion.

There is a mode in which libcurl is compiled with versioned symbols,
depending on the active SSL backend.

When multiple SSL backends are active, it does not make sense to favor
one over the others, so let's not: introduce a new prefix for the case
where multiple SSL backends are compiled into cURL.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Previously, we used as default SSL backend whatever was first in the
`available_backends` array.

However, some users may want to override that default without patching
the source code.

Now they can: with the --with-default-ssl-backend=<backend> option of
the ./configure script.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

When only one SSL backend is configured, it is totally unnecessary to
let multissl_init() configure the backend at runtime, we can select the
correct backend at build time already.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

To discern the active one from the inactive ones, put the latter into
parentheses.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

This new feature flag reports When cURL was built with multiple SSL
backends.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Previously, the code assumed that at most one of the SSL backends would
be compiled in, emulating OpenSSL's functions if the configured backend
was not OpenSSL itself.

However, now we allow building with multiple SSL backends and choosing
one at runtime. Therefore, metalink needs to be adjusted to handle this
scenario, too.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

The newly-introduced curl_global_sslset() function deserves to be
show-cased.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Let's add a compile time safe API to select an SSL backend. This
function needs to be called *before* curl_global_init(), and can be
called only once.

Side note: we do not explicitly test that it is called before
curl_global_init(), but we do verify that it is not called multiple times
(even implicitly).

If SSL is used before the function was called, it will use whatever the
CURL_SSL_BACKEND environment variable says (or default to the first
available SSL backend), and if a subsequent call to
curl_global_sslset() disagrees with the previous choice, it will fail
with CURLSSLSET_TOO_LATE.

The function also accepts an "avail" parameter to point to a (read-only)
NULL-terminated list of available backends. This comes in real handy if
an application wants to let the user choose between whatever SSL backends
the currently available libcurl has to offer: simply call

	curl_global_sslset(-1, NULL, &avail);

which will return CURLSSLSET_UNKNOWN_BACKEND and populate the avail
variable to point to the relevant information to present to the user.

Just like with the HTTP/2 push functions, we have to add the function
declaration of curl_global_sslset() function to the header file
*multi.h* because VMS and OS/400 require a stable order of functions
declared in include/curl/*.h (where the header files are sorted
alphabetically). This looks a bit funny, but it cannot be helped.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

There is information about the compiled-in SSL backends that is really
no concern of any code other than the SSL backend itself, such as which
function (if any) implements SHA-256 summing.

And there is information that is really interesting to the user, such as
the name, or the curl_sslbackend value.

Let's factor out the latter into a publicly visible struct. This
information will be used in the upcoming API to set the SSL backend
globally.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

When building software for the masses, it is sometimes not possible to
decide for all users which SSL backend is appropriate.

Git for Windows, for example,  uses cURL to perform clones, fetches and
pushes via HTTPS, and some users strongly prefer OpenSSL, while other
users really need to use Secure Channel because it offers
enterprise-ready tools to manage credentials via Windows' Credential
Store.

The current Git for Windows versions use the ugly work-around of
building libcurl once with OpenSSL support and once with Secure Channel
support, and switching out the binaries in the installer depending on
the user's choice.

Needless to say, this is a super ugly workaround that actually only
works in some cases: Git for Windows also comes in a portable form, and
in a form intended for third-party applications requiring Git
functionality, in which cases this "swap out libcurl-4.dll" simply is
not an option.

Therefore, the Git for Windows project has a vested interest in teaching
cURL to make the SSL backend a *runtime* option.

This patch makes that possible.

By running ./configure with multiple --with-<backend> options, cURL will
be built with multiple backends.

For the moment, the backend can be configured using the environment
variable CURL_SSL_BACKEND (valid values are e.g. "openssl" and
"schannel").

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>