Ensure schema qualification in pg_restore DISABLE/ENABLE TRIGGER commands.

Previously, this code blindly followed the common coding pattern of
passing PQserverVersion(AH->connection) as the server-version parameter
of fmtQualifiedId.  That works as long as we have a connection; but in
pg_restore with text output, we don't.  Instead we got a zero from
PQserverVersion, which fmtQualifiedId interpreted as "server is too old to
have schemas", and so the name went unqualified.  That still accidentally
managed to work in many cases, which is probably why this ancient bug went
undetected for so long.  It only became obvious in the wake of the changes
to force dump/restore to execute with restricted search_path.

In HEAD/v11, let's deal with this by ripping out fmtQualifiedId's server-
version behavioral dependency, and just making it schema-qualify all the
time.  We no longer support pg_dump from servers old enough to need the
ability to omit schema name, let alone restoring to them.  (Also, the few
callers outside pg_dump already didn't work with pre-schema servers.)

In older branches, that's not an acceptable solution, so instead just
tweak the DISABLE/ENABLE TRIGGER logic to ensure it will schema-qualify
its output regardless of server version.

Per bug #15338 from Oleg somebody.  Back-patch to all supported branches.

Discussion: https://postgr.es/m/153452458706.1316.5328079417086507743@wrigleys.postgresql.org

src/include/fe_utils/string_utils.h

@@ -25,8 +25,7 @@ extern PQExpBuffer (*getLocalPQExpBuffer) (void);
 
 /* Functions */
 extern const char *fmtId(const char *identifier);
-extern const char *fmtQualifiedId(int remoteVersion,
-			   const char *schema, const char *id);
+extern const char *fmtQualifiedId(const char *schema, const char *id);
 
 extern char *formatPGVersionNumber(int version_number, bool include_minor,
 					  char *buf, size_t buflen);