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/commands/user.c

@@ -1036,7 +1036,7 @@ DropRole(DropRoleStmt *stmt)
 /*
  * Rename role
  */
-void
+Oid
 RenameRole(const char *oldname, const char *newname)
 {
 	HeapTuple	oldtuple,
@@ -1142,6 +1142,8 @@ RenameRole(const char *oldname, const char *newname)
 	 * Close pg_authid, but keep lock till commit.
 	 */
 	heap_close(rel, NoLock);
+
+	return roleid;
 }
 
 /*