Adjust many backend functions to return OID rather than void.

Extracted from a larger patch by Dimitri Fontaine.  It is hoped that
this will provide infrastructure for enriching the new event trigger
functionality, but it seems possibly useful for other purposes as
well.

src/backend/catalog/pg_aggregate.c

@@ -42,7 +42,7 @@ static Oid lookup_agg_function(List *fnName, int nargs, Oid *input_types,
 /*
  * AggregateCreate
  */
-void
+Oid
 AggregateCreate(const char *aggName,
 				Oid aggNamespace,
 				Oid *aggArgTypes,
@@ -316,6 +316,8 @@ AggregateCreate(const char *aggName,
 		referenced.objectSubId = 0;
 		recordDependencyOn(&myself, &referenced, DEPENDENCY_NORMAL);
 	}
+
+	return procOid;
 }
 
 /*