Cleanups of the GSSAPI support. On Windows, standard GSS libraries

Cleanups of the GSSAPI support. On Windows, standard GSS libraries
are now loaded from standard locations (system32 for SSPI, the
registry-stored MIT KfW install location for KfW) rather than using
the risky default DLL search path; I've therefore also added an
option to manually specify a GSS DLL we haven't heard of (which
should in principle Just Work provided it supports proper GSS-API as
specified in the RFC). The same option exists on Unix too, because
it seemed like too useful an idea to reserve to Windows. In
addition, GSSAPI is now documented, and also (unfortunately) its GUI
configuration has been moved out into a sub-subpanel on the grounds
that it was too big to fit in Auth.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9003 cda61777-01e9-0310-a592-d414129be87e