This allows programs to include curl/urlapi.h directly.

Reviewed-by: Daniel Gustafsson
Reported-by: Ben Kohler
Fixes #3438
Closes #3441

Added CURLOPT_HTTP09_ALLOWED and --http0.9 for this purpose.

For now, both the tool and library allow HTTP/0.9 by default.
docs/DEPRECATE.md lays out the plan for when to reverse that default: 6
months after the 7.64.0 release. The options are added already now so
that applications/scripts can start using them already now.

Fixes #2873
Closes #3383

This adds the CURLOPT_TRAILERDATA and CURLOPT_TRAILERFUNCTION
options that allow a callback based approach to sending trailing headers
with chunked transfers.

The test server (sws) was updated to take into account the detection of the
end of transfer in the case of trailing headers presence.

Test 1591 checks that trailing headers can be sent using libcurl.

Closes #3350

Allows an application to pass in a pre-parsed URL via a URL handle.

Closes #3227

As has been outlined in the DEPRECATE.md document, the axTLS code has
been disabled for 6 months and is hereby removed.

Use a better supported TLS library!

Assisted-by: Daniel Gustafsson
Closes #3194

MesaLink support was added in commit 57348eb97d1b8fc3742e02c but the
backend was never added to the curl_sslbackend enum in curl/curl.h.
This adds the new backend to the enum and updates the relevant docs.

Closes #3195
Reviewed-by: Daniel Stenberg <daniel@haxx.se>

system.h selects the proper Sun settings when __SUNPRO_C is defined. The
Sun compiler does not define it when compiling C++ files.  I'm adding a
check also on __SUNPRO_CC to allow curl to work properly also when used
in a C++ project on Sun Solaris.

Closes #3181