GSSAPI: Improve documentation and tests

GSSAPI: Improve documentation and tests

The GSSAPI encryption patch neglected to update the protocol
documentation to describe how to set up a GSSAPI encrypted connection
from a client to the server, so fix that by adding the appropriate
documentation to protocol.sgml.

The tests added for encryption support were overly long and couldn't be
run in parallel due to race conditions; this was largely because each
test was setting up its own KDC to perform the tests.  Instead, merge
the authentication tests and the encryption tests into the original
test, where we only create one KDC to run the tests with.  Also, have
the tests check what the server's opinion is of the connection and if it
was GSS authenticated or encrypted using the pg_stat_gssapi view.

In passing, fix the libpq label for GSSENC-Mode to be consistent with
the "PGGSSENCMODE" environment variable.

Missing protocol documentation pointed out by Michael Paquier.
Issues with the tests pointed out by Tom Lane and Peter Eisentraut.

Refactored tests and added documentation by me.

Reviewed by Robbie Harwood (protocol documentation) and Michael Paquier
(rework of the tests).