Backport of subquery optimizations to 5.3.

There are still test failures because of:
- Wrong query results in outer join + semi join
- EXPLAIN output differences

sql/sql_prepare.cc

@@ -1353,6 +1353,7 @@ static int mysql_test_select(Prepared_statement *stmt,
     goto error;
 
   thd->used_tables= 0;                        // Updated by setup_fields
+  thd->thd_marker.emb_on_expr_nest= 0;
 
   /*
     JOIN::prepare calls