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_lex.h

@@ -604,6 +604,7 @@ public:
   List<TABLE_LIST> top_join_list; /* join list of the top level          */
   List<TABLE_LIST> *join_list;    /* list for the currently parsed join  */
   TABLE_LIST *embedding;          /* table embedding to the above list   */
+  List<TABLE_LIST> sj_nests;
   /*
     Beginning of the list of leaves in a FROM clause, where the leaves
     inlcude all base tables including view tables. The tables are connected
@@ -829,7 +830,7 @@ public:
   }
 
   void clear_index_hints(void) { index_hints= NULL; }
-
+  bool is_part_of_union() { return master_unit()->is_union(); }
 private:  
   /* current index hint kind. used in filling up index_hints */
   enum index_hint_type current_index_hint_type;