Reserve the "pg_" namespace for roles

This will prevent users from creating roles which begin with "pg_" and
will check for those roles before allowing an upgrade using pg_upgrade.

This will allow for default roles to be provided at initdb time.

Reviews by José Luis Tallón and Robert Haas

src/backend/commands/alter.c

@@ -747,6 +747,9 @@ ExecAlterOwnerStmt(AlterOwnerStmt *stmt)
 {
 	Oid			newowner = get_rolespec_oid(stmt->newowner, false);
 
+	check_rolespec_name(stmt->newowner,
+						"Cannot make reserved roles owners of objects.");
+
 	switch (stmt->objectType)
 	{
 		case OBJECT_DATABASE: