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/operatorcmds.c

@@ -59,7 +59,7 @@
  *
  * 'parameters' is a list of DefElem
  */
-void
+Oid
 DefineOperator(List *names, List *parameters)
 {
 	char	   *oprName;
@@ -295,20 +295,20 @@ DefineOperator(List *names, List *parameters)
 	/*
 	 * now have OperatorCreate do all the work..
 	 */
-	OperatorCreate(oprName,		/* operator name */
-				   oprNamespace,	/* namespace */
-				   typeId1,		/* left type id */
-				   typeId2,		/* right type id */
-				   functionOid, /* function for operator */
-				   commutatorName,		/* optional commutator operator name */
-				   negatorName, /* optional negator operator name */
-				   restrictionOid,		/* optional restrict. sel. procedure */
-				   joinOid,		/* optional join sel. procedure name */
-				   canMerge,	/* operator merges */
-				   canHash);	/* operator hashes */
+	return
+		OperatorCreate(oprName,		/* operator name */
+					   oprNamespace,	/* namespace */
+					   typeId1,		/* left type id */
+					   typeId2,		/* right type id */
+					   functionOid, /* function for operator */
+					   commutatorName,		/* optional commutator operator name */
+					   negatorName, /* optional negator operator name */
+					   restrictionOid,		/* optional restrict. sel. procedure */
+					   joinOid,		/* optional join sel. procedure name */
+					   canMerge,	/* operator merges */
+					   canHash);	/* operator hashes */
 }
 
-
 /*
  * Guts of operator deletion.
  */