Add FATAL_CLIENT_ONLY to ereport/elog

Add FATAL_CLIENT_ONLY to ereport/elog

SASL exchanges must end with either an AuthenticationOk or an
ErrorResponse from the server, and the standard way to produce an
ErrorResponse packet is for auth_failed() to call ereport(FATAL). This
means that there's no way for a SASL mechanism to suppress the server
log entry if the "authentication attempt" was really just a query for
authentication metadata, as is done with OAUTHBEARER.

Following the example of 1f9158ba4, add a FATAL_CLIENT_ONLY elevel. This
will allow ClientAuthentication() to choose not to log a particular
failure, while still correctly ending the authentication exchange before
process exit.

(The provenance of this patch is convoluted: since it's a mechanical
copy-paste of 1f9158ba4, both Zsolt Parragi and I produced nearly
identical versions independently, and Andrey Borodin reviewed Zsolt's
version. Tom Lane is the author of 1f9158ba4, but I don't want to imply
that he's signed off on this adaptation. See Discussion.)

Reviewed-by: Andrey Borodin <x4mmm@yandex-team.ru>
Discussion: https://postgr.es/m/CAN4CZFPim7hUiyb7daNKQPSZ8CvQRBGkVhbvED7yZi8VktSn4Q%40mail.gmail.com