new lock for multiupdate:

- open and create derived tables
- detect which tables should be locked for write
- lock and fill derived tables
some unitialized variables fixed

mysql-test/r/lock_multi.result

@@ -26,7 +26,7 @@ lock table t1 read;
  update t1,t2 set c=a where b=d;
 select c from t2;
 c
-1
+2
 drop table t1;
 drop table t2;
 create table t1 (a int);