In preparation for moving the NTLM message code into the SASL module,
and separating the native code from the SSPI code, added functions that
simply call the functions in curl_ntlm_msg.c.

In preparation for moving the NTLM message code into the SASL module
use the SASL functions in the HTTP code instead.

... so that non-configure builds get the correct functions too based on
the libssh2 version used.

... if the libssh2 version is new enough.

Bug: http://curl.haxx.se/bug/view.cgi?id=1448

Reworked the two sections that discuss modifying the Visual Studio pre-
processor settings, and vc6libcurl.dsw/vc6libcurl.dsp, to remove the
project files references as they have been superseded by a more thorough
set of project files for VC6 through VC12, but to also give the correct
reference to this setting in later versions of Visual Studio.

USE_NTLM would only be defined if: HTTP support was enabled, NTLM and
cryptography weren't disabled, and either a supporting cryptography
library or Windows SSPI was being compiled against.

This means it was not possible to build libcurl without HTTP support
and use NTLM for other protocols such as IMAP, POP3 and SMTP. Rather
than introduce a new SASL pre-processor definition, removed the HTTP
prerequisite just like USE_SPNEGO and USE_KRB5.

Note: Winbind support still needs to be dependent on CURL_DISABLE_HTTP
as it is only available to HTTP at present.

This bug dates back to August 2011 when I started to add support for
NTLM to SMTP.

Due to commit 40ee1ba0 the free in Curl_ntlm_decode_type2_target() is
longer required.

As the list has gotten a little messy and hard to read, especially with
the introduction of deprecated items, aligned the values and comments
into clean columns and reworked some of the comments in the process.

Fixed a couple of memory leaks as a result of moving code that used to
populate allocuserpwd and relied on it's clean up.

Following the addition of SSPI support for HTTP digest, synchronised
elements of the email digest code with that of the new HTTP code.

Post tidy up to ensure commonality of code style and variable names.

Technical difficulties prevented this from going into the
previous commit.

Added !SSPI to the features list of the HTTP digest tests, as SSPI
based builds now use the Windows SSPI messaging API rather than the
internal functions, and we can't control the random numbers that get
used as part of the digest.

Reworked the input token (challenge message) storage as what is passed
to the buf and desc in the response generation are typically blobs of
data rather than strings, so this is more in keeping with other areas
of the SSPI code, such as the NTLM message functions.

Added void reference to unused 'data' parameter back to fix compilation
warning.

Some versions of Microsoft's sspi.h don't define this.

Introduced in commit 7e6d51a7 these functions and definitions are only
required by the internal challenge-response functions now.

This temporarily breaks HTTP digest authentication in SSPI based builds,
causing CURLE_NOT_BUILT_IN to be returned. A follow up commit will
resume normal operation.