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/schemacmds.c

@@ -65,6 +65,10 @@ CreateSchemaCommand(CreateSchemaStmt *stmt, const char *queryString)
 	else
 		owner_uid = saved_uid;
 
+	/* Additional check to protect reserved role names */
+	check_rolespec_name(stmt->authrole,
+						"Cannot specify reserved role as owner.");
+
 	/* fill schema name with the user name if not specified */
 	if (!schemaName)
 	{