[PATCH v6] GSSAPI encryption support

Hello friends,

Here's yet another version of GSSAPI encryption support.  It's also
available for viewing on my github:

https://github.com/frozencemetery/postgres/tree/feature/gssencrypt6

Let me hit the highlights of this time around:

- Fallback code is back!  It's almost unchanged from early versions of
  this patchset.  Corresponding doc changes for this and the next item
  are of course included.

- Minor protocol change.  I did not realize that connection parameters
  were not read until after auth was complete, which means that in this
  version I go back to sending the AUTH_REQ_OK in the clear.  Though I
  found this initially irritating since it required re-working the
  should_crypto conditions, it ends up being a net positive since I can
  trade a library call for a couple variables.

- Client buffer flush on completion of authentication.  This should
  prevent the issue with the client getting unexpected message type of
  NUL due to encrypted data not getting decrypted.  I continue to be
  unable to replicate this issue, but since the codepath triggers in the
  "no data buffered case" all the math is sound.  (Famous last words I'm
  sure.)

- Code motion is its own patch.  This was requested and hopefully
  clarifies what's going on.

- Some GSSAPI authentication fixes have been applied.  I've been staring
  at this code too long now and writing this made me feel better.  If it
  should be a separate change that's fine and easy to do.

Thanks!