* use SSL/TLS where available
* follow permanent redirects

"At condition p_request, the value of p_request cannot be NULL."

Coverity CID 1306668.

... by removing the "do {} while (0)" block.

Coverity CID 1306669

use \fI-style instead of .BR for references

... to simplify checking when PUT _or_ POST have completed.

Reported-by: Frank Meier
Bug: http://curl.haxx.se/mail/lib-2015-06/0019.html

Some servers will request a client certificate, but not require one.
This change allows libcurl to connect to such servers when using
schannel as its ssl/tls backend. When a server requests a client
certificate, libcurl will now continue the handshake without one,
rather than terminating the handshake. The server can then decide
if that is acceptable or not. Prior to this change, libcurl would
terminate the handshake, reporting a SEC_I_INCOMPLETE_CREDENTIALS
error.

and a conversion to markdown. Removed the lib/README.* files. The idea
being to move toward having INTERNALS as the one and only "book" of
internals documentation.

Added a TOC to top of the document.

Although OpenSSL 1.1.0+ deprecated SSLv23_client_method in favor of
TLS_client_method LibreSSL and BoringSSL didn't and still use
SSLv23_client_method.

Bug: https://github.com/bagder/curl/commit/49a6642#commitcomment-11578009


Reported-by:  <asavah@users.noreply.github.com>

When CURL_SOCKET_BAD is returned in the callback, it should be treated
as an error (CURLE_COULDNT_CONNECT) if no other socket is subsequently
created when trying to connect to a server.

Bug: http://curl.haxx.se/mail/lib-2015-06/0047.html

- Try building a chain using issuers in the trusted store first to avoid
problems with server-sent legacy intermediates.

Prior to this change server-sent legacy intermediates with missing
legacy issuers would cause verification to fail even if the client's CA
bundle contained a valid replacement for the intermediate and an
alternate chain could be constructed that would verify successfully.

https://rt.openssl.org/Ticket/Display.html?id=3621&user=guest&pass=guest

Stop linking to the curl.haxx.se anchor pages, they are usually only
themselves pointers to the real page so better point there directly
instead.

The protocol is named "HTTP/2" after all. It is an alias for the
existing CURL_HTTP_VERSION_2_0 enum.

ERR_error_string_n() was introduced in 0.9.6, no need to #ifdef anymore

Code for OpenSSL 0.9.4 serves no purpose anymore!

It was present for OpenSSL 0.9.5 code but we only support 0.9.7 or
later.

The existing callback served no purpose.

Prior to this change any-domain cookies (cookies without a domain that
are sent to any domain) were exported with domain name "unknown".

Bug: https://github.com/bagder/curl/issues/292