[PATCH v7] GSSAPI encryption support

Hello friends,

New week, new version.  GitHub link:
https://github.com/frozencemetery/postgres/tree/feature/gssencrypt7

Changes in this version:

- Removed extra whitespace in auth code movement.

- Fixed connection desync issue.  A diff of this and v6 will reveal
  three issues:

  - First, that pg_GSS_read() didn't properly handle having a full
    buffer when called because pqsecure_raw_read() doesn't handle reads of
    size 0.  I've elected to change my own code here only, but it may be
    desirable to change pqsecure_raw_read() as well depending on whether
    other people are likely to hit that.

  - Second, that I was shunting data into the wrong buffer (don't know
    how this was overlooked; it has "write" right there in the name).

  - Third, that I'm now immediately decrypting that data into
    conn->inBuffer rather than deferring that step until later.  This
    removes the hang because now the connection will not erroneously get
    stuck polling while data is buffered.

Thanks!