Fix bugs in libpq's GSSAPI encryption support.

The critical issue fixed here is that if a GSSAPI-encrypted connection
is successfully made, pqsecure_open_gss() cleared conn->allow_ssl_try,
as an admittedly-hacky way of preventing us from then trying to tunnel
SSL encryption over the already-encrypted connection.  The problem
with that is that if we abandon the GSSAPI connection because of a
failure during authentication, we would not attempt SSL encryption
in the next try with the same server.  This can lead to unexpected
connection failure, or silently getting a non-encrypted connection
where an encrypted one is expected.

Fortunately, we'd only manage to make a GSSAPI-encrypted connection
if both client and server hold valid tickets in the same Kerberos
infrastructure, which is a relatively uncommon environment.
Nonetheless this is a very nasty bug with potential security
consequences.  To fix, don't reset the flag, instead adding a
check for conn->gssenc being already true when deciding whether
to try to initiate SSL.

While here, fix some lesser issues in libpq's GSSAPI code:

* Use the need_new_connection stanza when dropping an attempted
GSSAPI connection, instead of partially duplicating that code.
The consequences of this are pretty minor: AFAICS it could only
lead to auth_req_received or password_needed remaining set when
they shouldn't, which is not too harmful.

* Fix pg_GSS_error() to not repeat the "mprefix" it's given multiple
times, and to notice any failure return from gss_display_status().

* Avoid gratuitous dependency on NI_MAXHOST in
pg_GSS_load_servicename().

Per report from Mikael Gustavsson.  Back-patch to v12 where
this code was introduced.

Discussion: https://postgr.es/m/e5b0b6ed05764324a2f3fe7acfc766d5@smhi.se

src/interfaces/libpq/fe-connect.c

@@ -2814,11 +2814,16 @@ keep_going:						/* We will come back to here until there is
 #ifdef USE_SSL
 
 				/*
-				 * If SSL is enabled and we haven't already got it running,
-				 * request it instead of sending the startup message.
+				 * If SSL is enabled and we haven't already got encryption of
+				 * some sort running, request SSL instead of sending the
+				 * startup message.
 				 */
 				if (conn->allow_ssl_try && !conn->wait_ssl_try &&
-					!conn->ssl_in_use)
+					!conn->ssl_in_use
+#ifdef ENABLE_GSS
+					&& !conn->gssenc
+#endif
+					)
 				{
 					ProtocolVersion pv;
 
@@ -2947,6 +2952,7 @@ keep_going:						/* We will come back to here until there is
 						}
 						/* Otherwise, proceed with normal startup */
 						conn->allow_ssl_try = false;
+						/* We can proceed using this connection */
 						conn->status = CONNECTION_MADE;
 						return PGRES_POLLING_WRITING;
 					}
@@ -3044,8 +3050,7 @@ keep_going:						/* We will come back to here until there is
 						 * don't hang up the socket, though.
 						 */
 						conn->try_gss = false;
-						pqDropConnection(conn, true);
-						conn->status = CONNECTION_NEEDED;
+						need_new_connection = true;
 						goto keep_going;
 					}
 
@@ -3063,6 +3068,7 @@ keep_going:						/* We will come back to here until there is
 						}
 
 						conn->try_gss = false;
+						/* We can proceed using this connection */
 						conn->status = CONNECTION_MADE;
 						return PGRES_POLLING_WRITING;
 					}
@@ -3091,8 +3097,7 @@ keep_going:						/* We will come back to here until there is
 					 * the current connection to do so, though.
 					 */
 					conn->try_gss = false;
-					pqDropConnection(conn, true);
-					conn->status = CONNECTION_NEEDED;
+					need_new_connection = true;
 					goto keep_going;
 				}
 				return pollres;
@@ -3263,10 +3268,9 @@ keep_going:						/* We will come back to here until there is
 					 */
 					if (conn->gssenc && conn->gssencmode[0] == 'p')
 					{
-						/* postmaster expects us to drop the connection */
+						/* only retry once */
 						conn->try_gss = false;
-						pqDropConnection(conn, true);
-						conn->status = CONNECTION_NEEDED;
+						need_new_connection = true;
 						goto keep_going;
 					}
 #endif