[SHOW] EXPLAIN UPDATE/DELETE, code re-structuring

- Make query plan be re-saved after the first join execution
  (saving it after JOIN::cleanup is too late because EXPLAIN output 
   is currently produced before that)
- Handle QPF allocation/deallocation for edge cases, like unsuccessful
  BINLOG command.
- Work around the problem with UNION's direct subselects not being visible.
- Update test results ("Using temporary; Using filesort" are now always printed 
  last in the Extra column)
- This cset gets rid of memory leaks/crashes. Some result mismatches still remain.

sql/sql_select.h

@@ -1336,7 +1336,10 @@ public:
     pre_sort_join_tab= NULL;
     emb_sjm_nest= NULL;
     sjm_lookup_tables= 0;
+
+    exec_qpf_saved= false;
   }
+  bool exec_qpf_saved;
 
   int prepare(Item ***rref_pointer_array, TABLE_LIST *tables, uint wind_num,
 	      COND *conds, uint og_num, ORDER *order, ORDER *group,