...and some minor code style changes.

As this is covered by the PROTOCOLS section and saves having to update
two parts of the document with the same information in future.

As implementations are refereed to GSS-API libraries as per the RFC and
GSSAPI typically refers to an authentication mechanism.

Added missing IMAP to the protocol list.

...as this option affects more that just FTP.

Whilst the description included information about SMTP, the protocol
list only showed "TTP, FTP, IMAP, POP3".

... and allow building with nghttp2 but completely without NPN and ALPN,
as nghttp2 can still be used for plain-text HTTP.

Reported-by: Lucas Pardue

... since the conditional in the code are now based on OpenSSL versions
instead to better support non-configure builds.

conversion from 'size_t' to 'unsigned int', possible loss of data

Return a more appropriate error, rather than CURLE_OUT_OF_MEMORY when
acquiring the credentials handle fails. This is then consistent with
the code prior to commit f7e24683 when log-in credentials were empty.

Fixed the ability to use the current log-in credentials with DIGEST-MD5.
I had previously disabled this functionality in commit 607883f1 as I
couldn't get this to work under Windows 8, however, from testing HTTP
Digest authentication through Windows SSPI and then further testing of
this code I have found it works in Windows 7.

Some further investigation is required to see what the differences are
between Windows 7 and 8, but for now enable this functionality as the
code will return an error when AcquireCredentialsHandle() fails.

Its last use has been removed by the previous commit.

This code was already deactivated by commit
ec783dc1.

This check works also with to non-configure platforms.

HTTP 1.1 is clearly specified to only allow three digit response codes,
and libcurl used sscanf("%3d") for that purpose. This made libcurl
support smaller numbers but not larger. It does now, but we will not
make any specific promises nor document this further since it is going
outside of what HTTP is.

Bug: http://curl.haxx.se/bug/view.cgi?id=1441
Reported-by: Balaji

It has not been used since commit f7bfdbab in 2011

Continuing commit 0eb3d15c more return code variable name changes.

Continuing commit 0eb3d15c more return code variable name changes.

Code cleanup to try and synchronise code between the different SSPI
based authentication mechanisms.

Code cleanup to try and synchronise code between the different SSPI
based authentication mechanisms.