sspi: Minor code tidy up to standardise coding style

Following the recent changes and in attempt to align the SSPI based
authentication code performed the following:

* Use NULL and SECBUFFVERSION rather than hard coded constants.
* Avoid comparison of zero in if statements.
* Standardised the buf and desc setup code.
docs: Update SPNEGO and GSS-API related doc sections

Reflect recent changes in SPNEGO and GSS-API code in the docs.
Update them with appropriate namings and remove visible spots for
GSS-Negotiate.