SSPI authentication - patch

Attached is the patch to support SSPI authentication in libpq. With this
patch, I can authenticate windows clients against a linux server using
kerberos *without* reqiring setting up MIT kerberos on the windows side.
Protocol has not changed at all.

For now, it's implemented as mutually exclusive with the MIT version. This
can of course be changed, but I have yet to see a good way how to expose
the difference to the app.

By setting it up the way it is, some code is shared between GSSAPI and
SSPI, which is also why the SSPI functions are just different versions of
the pg_GSS_startup/continue functions. If we want to support both, those
will of course have to be changed.

I have not set up autoconf for it yet, pending the outcome of the
discussion about what should be default. What in envision doing there is
the same as what I've done for MSVC so far - enable SSPI unless
--enable-gss is specified, in which case SSPI is disabled and GSSAPI is
enabled instead.

I'll be working on the server side as well ;-), but getting libpq is the
most important part of it.

Comments? If there are none, I'll apply this version soon.

//Magnus