[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_lex.cc

@@ -2552,7 +2552,8 @@ void Query_tables_list::destroy_query_tables_list()
 */
 
 LEX::LEX()
-  :result(0), option_type(OPT_DEFAULT), is_lex_started(0),
+  : query_plan_footprint(NULL),
+    result(0), option_type(OPT_DEFAULT), is_lex_started(0),
    limit_rows_examined_cnt(ULONGLONG_MAX)
 {