[PATCH v20] GSSAPI encryption support

Hello friends,

Attached please find version 20 of the GSSAPI encryption support.  This
has been rebased onto master (thanks Stephen for calling out ab69ea9).
Other changes since v19 from Stephen's review:

- About 100 lines of new comments

- pgindent run over code (only the stuff I'm changing; it reports other
  problems on master, but if I understand correctly, that's not on me to
  fix here)

Thanks for the feedback!  And speaking of feedback, this patch is in the
"Needs Review" state so if people have additional feedback, that would
be appreciated.  I've CC'd people who I can remember reviewing before;
they should of course feel no obligation to review again if time
commitments/interests do not allow.

Thanks,
--Robbie