Combine options for RangeVarGetRelidExtended() into a flags argument.

A followup patch will add a SKIP_LOCKED option. To avoid introducing
evermore arguments, breaking existing callers each time, introduce a
flags argument. This'll no doubt break a few external users...

Also change the MISSING_OK behaviour so a DEBUG1 debug message is
emitted when a relation is not found.

Author: Nathan Bossart
Reviewed-By: Michael Paquier and Andres Freund
Discussion: https://postgr.es/m/20180306005349.b65whmvj7z6hbe2y@alap3.anarazel.de

src/backend/commands/trigger.c

@@ -1667,7 +1667,7 @@ renametrig(RenameStmt *stmt)
 	 * release until end of transaction).
 	 */
 	relid = RangeVarGetRelidExtended(stmt->relation, AccessExclusiveLock,
-									 false, false,
+									 0,
 									 RangeVarCallbackForRenameTrigger,
 									 NULL);