A problem would occur if the entire buffer couldn't be sent at once and
it could cause an infinte POST loop.

Reported-by: vfloyd6 on github
Fixes #2947

Add functionality so that protocols can do custom keepalive on their
connections, when an external API function is called.

Add docs for the new options in 7.62.0

Closes #1641

Sometimes it may be considered a security risk to load an external
OpenSSL configuration automatically inside curl_global_init(). The
configuration option --disable-ssl-auto-load-config disables this
automatism. The Windows build scripts winbuild/Makefile.vs provide a
corresponding option ENABLE_SSL_AUTO_LOAD_CONFIG accepting a boolean
value.

Setting neither of these options corresponds to the previous behavior
loading the external OpenSSL configuration automatically.

Fixes #2724
Closes #2791

The gcc typecheck macros and coverity combined made it warn on the 2nd
argument for ERROR_CHECK_SETOPT(). Here's minor rearrange to please it.

Coverity CID 1439115 and CID 1439114.

SEC_E_APPLICATION_PROTOCOL_MISMATCH isn't defined in some versions of
mingw and would require an ifdef otherwise.

Reported-by: Thomas Glanzmann
Approved-by: Marc Hörsken
Bug: https://curl.haxx.se/mail/lib-2018-09/0020.html
Closes #2950

... and add "MAILINDEX".

As described in #2789, this is a suggested solution.  Changing UID=xx to
actually get mail with UID xx and add "MAILINDEX" to get a mail with a
special index in the mail box (old behavior).  So MAILINDEX=1 gives the
first non deleted mail in the mail box.

Fixes #2789
Closes #2815

This is step 3 of #2888.

Fixes #2888
Closes #2896