Change how we handle the Ssh_gss_buf type. Previously, we defined it

Change how we handle the Ssh_gss_buf type.  Previously, we defined it
ourselves, but on Unix then assumed it was compatible with the system's
gss_buffer_desc, which wasn't the case on LP64 systems.  Now, on Unix
we make Ssh_gss_buf into an alias for gss_buffer_desc, though we keep
something similar to the existing behaviour on Windows.  This requires
renaming a couple of the fields in Ssh_gss_buf, and hence fixing all
the references.

Tested on Linux (MIT Kerberos) and Solaris.  Compiled on NetBSD (Heimdal).
Not tested on Windows because neither mingw32 nor winegcc worked out of the
box for me.  I think the Windows changes are all syntactic, though, so
if this compiles it should work no worse than before.

[originally from svn r8326]