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/view.c

@@ -420,7 +420,7 @@ UpdateRangeTableOfViewParse(Oid viewOid, Query *viewParse)
  * DefineView
  *		Execute a CREATE VIEW command.
  */
-void
+Oid
 DefineView(ViewStmt *stmt, const char *queryString)
 {
 	Query	   *viewParse;
@@ -540,4 +540,6 @@ DefineView(ViewStmt *stmt, const char *queryString)
 	 * Now create the rules associated with the view.
 	 */
 	DefineViewRules(viewOid, viewParse, stmt->replace);
+
+	return viewOid;
 }