Closes #1868

- The part kind MIMEKIND_FILE and associated code are suppressed.
- Seek data origin offset not used anymore: suppressed.
- MIMEKIND_NAMEDFILE renamed MIMEKIND_FILE; associated fields/functions
  renamed accordingly.
- Curl_getformdata() processes stdin via a callback.

Back in 2008, (and commit 3f3d6ebe) we changed the logic in how we
determine the native type for `curl_off_t`. To really make sure we
didn't break ABI without bumping SONAME, we introduced logic that
attempted to detect that it would use a different size and thus not be
compatible. We also provided a manual switch that allowed users to tell
configure to bump SONAME by force.

Today, we know of no one who ever got a SONAME bump auto-detected and we
don't know of anyone who's using the manual bump feature. The auto-
detection is also no longer working since we introduced defining
curl_off_t in system.h (7.55.0).

Finally, this bumping logic is not present in the cmake build.

Closes #1861

- Do a case-insensitive comparison of CURL_SSL_BACKEND env as well.

- Change Curl_strcasecompare calls to strcasecompare
  (maps to the former but shorter).

Follow-up to c290b8fb.

Bug: https://github.com/curl/curl/commit/c290b8f#commitcomment-24094313

Co-authored-by: Jay Satiro

This is an adaptation of 2 of Peter Wu's SSLKEYLOGFILE implementations.

The first one, written for old OpenSSL versions:
https://git.lekensteyn.nl/peter/wireshark-notes/tree/src/sslkeylog.c

The second one, written for BoringSSL and new OpenSSL versions:
https://github.com/curl/curl/pull/1346

Note the first one is GPL licensed but the author gave permission to
waive that license for libcurl.

As of right now this feature is disabled by default, and does not have
a configure option to enable it. To enable this feature define
ENABLE_SSLKEYLOGFILE when building libcurl and set environment
variable SSLKEYLOGFILE to a pathname that will receive the keys.

And in Wireshark change your preferences to point to that key file:
Edit > Preferences > Protocols > SSL > Master-Secret

Co-authored-by: Peter Wu

Ref: https://github.com/curl/curl/pull/1030
Ref: https://github.com/curl/curl/pull/1346

Closes https://github.com/curl/curl/pull/1866

mime_state is now a typedef.

curl_mime_encoder() is operational and documented.
curl tool -F option is extended with ";encoder=".
curl tool --libcurl option generates calls to curl_mime_encoder().
New encoder tests 648 & 649.
Test 1404 extended with an encoder specification.

... and slightly edited to follow our code style better.

Up2date versions of OpenSSL maintain the default reasonably secure
without breaking compatibility, so it is better not to override the
default by curl.  Suggested at https://bugzilla.redhat.com/1483972

Closes #1846

...for curl-config and its corresponding test 1014

Added test 1904 to verify.

Reported-by: Lawrence Wagerfield
Fixes #1859
Closes #1860

... to a README file that doesn't exist anymore

and some minor whitespace fixes

... not *CACERT_BADFILE as it isn't really because of a bad file.

Bug: https://curl.haxx.se/mail/lib-2017-09/0002.html
Closes #1858

... since people copy and paste code from this example and thus they get
an inefficient POST operation without a good reason and sometimes
without understanding why.

Instead this now returns as much data as possible.

... as the other functions. Makes it easier to machine-parse!

As it was added to multi.h simply to not break test 1135, which now has
been disabled due to the mime API addition anyway and su we can now move
the sslset stuff to where the other curl_global_* prototypes are.

Use and generate CURL_ZERO_TERMINATED in curl tool and tests.

To support telling a string is nul-terminated, symbol CURL_ZERO_TERMINATED
has been introduced.

Documentation updated accordingly.

symbols in versions updated. Added form API symbols deprecation info.

This feature is badly supported in Windows: as a replacement, a caller has
to use curl_mime_data_cb() with fread, fseek and possibly fclose
callbacks to process opened files.

The cli tool and documentation are updated accordingly.

The feature is however kept internally for form API compatibility, with
the known caveats it always had.

As a side effect, stdin size is not determined by the cli tool even if
possible and this results in a chunked transfer encoding. Test 173 is
updated accordingly.

Additional mime-specific tests.
Existing tests updated to reflect small differences (Expect: 100-continue,
data size change due to empty lines, etc).
Option -F headers= keyword added to tests.
test1135 disabled until the entry point order change is resolved.
New example smtp-mime.
Examples postit2 and multi-post converted from form API to mime API.

Extended -F option syntax to support multipart mail messages.
-F keyword headers= added to include custom headers in parts.
Documentation upgraded.