libpq: Bail out during SSL/GSS negotiation errors

libpq: Bail out during SSL/GSS negotiation errors

This commit changes libpq so that errors reported by the backend during
the protocol negotiation for SSL and GSS are discarded by the client, as
these may include bytes that could be consumed by the client and write
arbitrary bytes to a client's terminal.

A failure with the SSL negotiation now leads to an error immediately
reported, without a retry on any other methods allowed, like a fallback
to a plaintext connection.

A failure with GSS discards the error message received, and we allow a
fallback as it may be possible that the error is caused by a connection
attempt with a pre-11 server, GSS encryption having been introduced in
v12.  This was a problem only with v17 and newer versions; older
versions discard the error message already in this case, assuming a
failure caused by a lack of support for GSS encryption.

Author: Jacob Champion
Reviewed-by: Peter Eisentraut, Heikki Linnakangas, Michael Paquier
Security: CVE-2024-10977
Backpatch-through: 12