To avoid urldata.h being included from the header file or that the
source file has the correct include order as highlighted by one of
the auto builds recently.

warning: no previous prototype for 'Curl_sasl_create_digest_md5_message'

warning: 'sasl_digest_get_key_value' defined but not used

* Added comments to SSPI NTLM message generation
* Added comments to native DIGEST-MD5 code
* Removed redundant identity pointer

When CURL_DISABLE_CRYPTO_AUTH is defined the DIGEST-MD5 code should not
be included, regardless of whether USE__WINDOWS_SSPI is defined or not.
This is indicated by the definition of USE_HTTP_NEGOTIATE and USE_NTLM
in curl_setup.h.

* Added information regarding the February 2003 Platform SDK for VC6
* Updated the introduction to be similar to the IDE projects README

Added a more thorough version of the VC8 project files that exist in
the "vs" folder with the intention to add support for other versions of
Visual Studio. These files support side-by-side compilation, 32-bit and
64-bit builds as well as support for some of the third-party libraries
curl uses.

And clarify for curl that --proxy-header now must be used for headers
that are meant for a proxy, and they will not be included if the request
is not for a proxy.

Updated the docs to clarify and the code accordingly, with test 1528 to
verify:

When CURLHEADER_SEPARATE is set and libcurl is asked to send a request
to a proxy but it isn't CONNECT, then _both_ header lists
(CURLOPT_HTTPHEADER and CURLOPT_PROXYHEADER) will be used since the
single request is then made for both the proxy and the server.

... and sorted the list

Modified the logic so that CURLOPT_HEADEROPT now controls if PROXYHEADER
is actually used or not.

Includes docs and new test cases: 1525, 1526 and 1527

Co-written-by: Vijay Panghal

Without request body there's no point in asking for 100-continue.

Bug: http://curl.haxx.se/bug/view.cgi?id=1349
Reported-by: JimS

When doing passive FTP, the multi state function needs to extract and
use the happy eyeballs sockets to wait for to check for completion!

Bug: http://curl.haxx.se/mail/lib-2014-02/0135.html (ruined)
Reported-by: Alan

Since all present tests now have <keywords> listed, this script will now
refuse to run a given test case if no such section is provided.
Hopefully this will help us make sure new test cases get keywords added
at start.

In addition to commit fe260b75 fixed the same issue for RFC-821 based
SMTP servers and allow the credientials to be given to curl even though
they are not used with the server.