standby promote: fall back to "pg_ctl promote" if necessary

From PostgreSQL 12, the SQL-level function "pg_promote()" can be used
to promote a PostgreSQL instance, however usage is restricted to
superusers and users to whom explicit execution permission for this
function has been granted.

Therefore, if execution permission is not available, fall back to
"pg_ctl promote".

dbutils.h

@@ -441,6 +441,7 @@ uint64		system_identifier(PGconn *conn);
 TimeLineHistoryEntry *get_timeline_history(PGconn *repl_conn, TimeLineID tli);
 
 /* user/role information functions */
+bool		can_execute_pg_promote(PGconn *conn);
 bool		connection_has_pg_settings(PGconn *conn);
 bool		is_replication_role(PGconn *conn, char *rolname);
 bool		is_superuser_connection(PGconn *conn, t_connection_user *userinfo);