Detect support for compiler symbol visibility flags and apply those
according to CURL_HIDDEN_SYMBOLS option.
It should work true to the autotools build except it tries to unhide
symbols on Windows when requested and prints warning if it fails.

Ref: https://github.com/curl/curl/issues/981#issuecomment-242665951
Reported-by: Daniel Stenberg

... by partially reverting f975f060. The allocation could be made by
OpenSSL so the free must be made with OPENSSL_free() to avoid problems.

Reported-by: Harold Stuart
Fixes #1005

... by making sure we don't count down the "upload left" counter when the
uploaded size is unknown and then it can be allowed to continue forever.

Fixes #996

Since we're using CURLE_FTP_WEIRD_SERVER_REPLY in imap, pop3 and smtp as
more of a generic "failed to parse" introduce an alias without FTP in
the name.

Closes https://github.com/curl/curl/pull/975

... as that function slipped through once before.

This hash is used to verify the original downloaded certificate bundle
and also included in the generated bundle's comment header. Also
rename related internal symbols to algorithm-agnostic names.

CURLINFO_SSL_VERIFYRESULT does not get the certificate verification
result when SSL_connect fails because of a certificate verification
error.

This fix saves the result of SSL_get_verify_result so that it is
returned by CURLINFO_SSL_VERIFYRESULT.

Closes https://github.com/curl/curl/pull/995

While noErr and errSecSuccess are defined as the same value, the API
documentation states that SecPKCS12Import() returns errSecSuccess if
there were no errors in importing. Ensure that a future change of the
defined value doesn't break (however unlikely) and be consistent with
the API docs.

With OPENSSL_API_COMPAT=0x10100000L (OpenSSL 1.1 API), the cleanup
functions are unavailable (they're no-ops anyway in OpenSSL 1.1). The
replacements for SSL_load_error_strings, SSLeay_add_ssl_algorithms, and
OpenSSL_add_all_algorithms are called automatically [1][2]. SSLeay() is
now called OpenSSL_version_num().

[1]: https://www.openssl.org/docs/man1.1.0/ssl/OPENSSL_init_ssl.html
[2]: https://www.openssl.org/docs/man1.1.0/crypto/OPENSSL_init_crypto.html

Closes #992

Fixes #982

CPPPFLAGS is now CPPPFLAG. Fixes CURL_CHECK_DEF.

Fixes #958

Speed limits (from CURLOPT_MAX_RECV_SPEED_LARGE &
CURLOPT_MAX_SEND_SPEED_LARGE) were applied simply by comparing limits
with the cumulative average speed of the entire transfer; While this
might work at times with good/constant connections, in other cases it
can result to the limits simply being "ignored" for more than "short
bursts" (as told in man page).

Consider a download that goes on much slower than the limit for some
time (because bandwidth is used elsewhere, server is slow, whatever the
reason), then once things get better, curl would simply ignore the limit
up until the average speed (since the beginning of the transfer) reached
the limit.  This could prove the limit useless to effectively avoid
using the entire bandwidth (at least for quite some time).

So instead, we now use a "moving starting point" as reference, and every
time at least as much as the limit as been transferred, we can reset
this starting point to the current position. This gets ...

Especially in regards to the multiplexing part.

* Added description to Curl_sspi_free_identity()
* Added parameter and return explanations to Curl_sspi_global_init()
* Added parameter explaination to Curl_sspi_global_cleanup()

Missed from commit 8356022d.

...and discuss a possible solution.

CURLDEBUG is for the memory debugging

DEBUGBUILD is for the extra debug stuff

Pointed-out-by: Steve Holme

darwinssl: add documentation stating that the --cainfo option is intended for backward compatibility only

In other news, I changed one other reference to "Mac OS X" in the documentation (that I previously wrote) to say "macOS" instead.

Follow-up to c3e906e9, seems like a more appropriate error code

Suggested-by: Jay Satiro

Fixes #986

With HTTP/2 each transfer is made in an indivial logical stream over the
connection, making most previous errors that caused the connection to get
forced-closed now instead just kill the stream and not the connection.

Fixes #941

... instead of if() before the switch(), add a default to the switch so
that the compilers don't warn on "warning: enumeration value
'PLATFORM_DONT_CARE' not handled in switch" anymore.

- Disable ALPN on Wine.

- Don't pass input secbuffer when ALPN is disabled.

When ALPN support was added a change was made to pass an input secbuffer
to initialize the context. When ALPN is enabled the buffer contains the
ALPN information, and when it's disabled the buffer is empty. In either
case this input buffer caused problems with Wine and connections would
not complete.

Bug: https://github.com/curl/curl/issues/983
Reported-by: Christian Fillion