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project files references as they have been superseded by a more thorough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.

Reported-by: Christian Hägele
Bug: http://curl.haxx.se/mail/lib-2014-11/0078.html

docs/THANKS-filter is a new filter file for converting contributor names
we get or have recorded in alternative formats to the one we already use
in THANKS. To help us show individual contributors using a single
presentation of their names.

The removed names also appear as:
Andrés García, François Charlier, Gökhan Şengün, Michał Górny, Sébastien
Willemijns, Christopher Conroy, John E. Malmberg, Luca Altea, Peter Su,
S. Moonesamy, Samuel Listopad, Yasuharu Yamada, Karl Moerder

./contributors.sh found these extra ones that somehow had fallen
through the cracks and never gotten added here.

Reported-by: Frank Gevaerts

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

...and minor rewording on the same paragraph.

... instead of duplicating info.

...and removed duplication of what protocols are supported from the
description text.

...and revised the proxy wording a little as well.

...and corrected a related typo in curl_easy_setopt.3.

As this has been present for SOCKSv5 proxy since v7.19.4 and for IMAP,
POP3 and SMTP authentication since v7.38.0.

Support for Kerberos V4 was removed in v7.33.0.

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".

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.