Improve behavior of concurrent ALTER TABLE, and do some refactoring.

ALTER TABLE (and ALTER VIEW, ALTER SEQUENCE, etc.) now use a
RangeVarGetRelid callback to check permissions before acquiring a table
lock.  We also now use the same callback for all forms of ALTER TABLE,
rather than having separate, almost-identical callbacks for ALTER TABLE
.. SET SCHEMA and ALTER TABLE .. RENAME, and no callback at all for
everything else.

I went ahead and changed the code so that no form of ALTER TABLE works
on foreign tables; you must use ALTER FOREIGN TABLE instead.  In 9.1,
it was possible to use ALTER TABLE .. SET SCHEMA or ALTER TABLE ..
RENAME on a foreign table, but not any other form of ALTER TABLE, which
did not seem terribly useful or consistent.

Patch by me; review by Noah Misch.

src/backend/commands/alter.c

@@ -192,8 +192,7 @@ ExecAlterObjectSchemaStmt(AlterObjectSchemaStmt *stmt)
 		case OBJECT_TABLE:
 		case OBJECT_VIEW:
 		case OBJECT_FOREIGN_TABLE:
-			AlterTableNamespace(stmt->relation, stmt->newschema,
-								stmt->objectType, AccessExclusiveLock);
+			AlterTableNamespace(stmt);
 			break;
 
 		case OBJECT_TSPARSER: