Add WITH_MBEDTLS option. Make WITH_SSL, WITH_MBEDTLS and ENABLE_WINSSL
options mutual exclusive.

Closes #606

As of commit d9f3b365

These configure vars are modified in a curl-specific way but never
evaluated or loaded from cache, even though they are designated as
_cv_. We could either implement proper AC_CACHE_CHECKs for them, or
remove them completely.

Fixes #603 as ac_cv_func_gethostbyname is no longer clobbered, and
AC_CHECK_FUNC(gethostbyname...) will no longer spuriously succeed after
the first configure run with caching.

`ac_cv_func_strcasecmp` is curious, see #770.

`eval "ac_cv_func_$func=yes"` can still cause problems as it works in
tandem with AC_CHECK_FUNCS and then potentially modifies its result. It
would be best to rewrite this test to use a new CURL_CHECK_FUNCS macro,
which works the same as AC_CHECK_FUNCS but relies on caching the values
of curl_cv_func_* variables, without modifiying ac_cv_func_*.

These configure vars are modified in a curl-specific way and modified by
the configure process, but are never loaded from cache, even though they
are designated as _cv_. We should implement proper AC_CACHE_CHECKs for
them eventually.

This variable must not be cached in its current form, as any cached
information will prevent the next configure run from determining the
correct LIBS needed for the function. Thus, rename prefix `ac_cv_` to
just `curl_`.

This was automated by:

sed -b -i -f <(ack -A1 AC_CACHE_CHECK | \
               ack -o 'ac_cv_.*?\b' | \
               sort -u | xargs -n1 bash -c \
                    'echo "s/$0/curl_cv_${0#ac_cv_}/g"') \
    $(git ls-files)

This only changed the prefix for 16 variables actually checked with
AC_CACHE_CHECK.

The RSA, DSA and DH structs are now opaque and require use of new APIs

Fixes #763

Fixed commit fa5fa65a to not use NULLs in if condition.

At ConnectionExists, both check->proxyuser and check->proxypasswd
could be NULL, so make sure to check first.

Fixes #765

... for checking ability to receive full HTTP response when POST request
is used with slow read callback function.

This test checks for bug #657 and verifies the work-around from
72d5e144.

Closes #720

WinSock destroys recv() buffer if send() is failed. As result - server
response may be lost if server sent it while curl is still sending
request. This behavior noticeable on HTTP server short replies if
libcurl use several send() for request (usually for POST request).
To workaround this problem, libcurl use recv() before every send() and
keeps received data in intermediate buffer for further processing.

Fixes: #657
Closes: #668

This commit fixes a Clang warning introduced in curl-7_48_0-190-g8f72b13:

Error: CLANG_WARNING:
lib/connect.c:1120:11: warning: The right operand of '==' is a garbage value
1118|       }
1119|
1120|->     if(-1 == rc)
1121|         error = SOCKERRNO;
1122|     }

... as checksrc now skips comments

... that the new checksrc detected

... since checksrc now skips comments

... but output non-stripped version of the line, even if that then can
make the script identify the wrong position in the line at
times. Showing the line stripped (ie without comments) is just too
surprising.

... to make it easier to add lines in patches that won't just break all
other patches trying to add lines too.

(since we just merged two major features)

Closes #660

'strncasecmp' was once provided by libresolv (no trailing e) for SunOS,
but this check is broken and most likely adds nothing useful. Removing
now.

Reported-by: Irfan Adilovic

Discussed in #770

Reported-by: bodop

Fixes #771

... to properly support out of source tree builds.

Makes curl connect to the given host+port instead of the host+port found
in the URL.

To allow both release and debug builds in parallel.

Reported-by: Rod Widdowson

Fixes #769

Ref: https://github.com/curl/curl/issues/659
Ref: https://github.com/curl/curl/pull/663

- Error if a header line is larger than supported.

- Warn if cumulative header line length may be larger than supported.

- Allow spaces when parsing the path component.

- Make sure each header line ends in \r\n. This fixes an out of bounds.

- Disallow header continuation lines until we decide what to do.

Ref: https://github.com/curl/curl/issues/659
Ref: https://github.com/curl/curl/pull/663

Ref: https://github.com/curl/curl/issues/659
Ref: https://github.com/curl/curl/pull/663