[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!