Moving max/min optimization from prepare to optimization phase. MWL#148

mysql-test/r/explain.result:
  fixed results (new item)
mysql-test/r/subselect.result:
  fixed results (new item)
mysql-test/r/subselect_no_mat.result:
  fixed results (new item)
mysql-test/r/subselect_no_opts.result:
  fixed results (new item)
mysql-test/r/subselect_no_semijoin.result:
  Fixed results (new item)
mysql-test/suite/pbxt/r/subselect.result:
  Fixed results (new item)
mysql-test/t/explain.test:
  Fixed results (correct behaviour)
sql/item_cmpfunc.cc:
  Pass through  for max/min
sql/item_subselect.cc:
  moving max/min
sql/item_subselect.h:
  moving max/min
sql/mysql_priv.h:
  new uncacheble flags added
sql/opt_subselect.cc:
  maxmin moved.
sql/opt_subselect.h:
  New function for maxmin.
sql/sql_class.h:
  debug code
sql/sql_lex.cc:
  Fixed flags.
  Limit setting fixed.
sql/sql_lex.h:
  2 new flags.
sql/sql_select.cc:
  Prepare divided on 2 function to be able recollect some info after transformation.
sql/sql_select.h:
  Prepare divided on 2 functions.

sql/sql_select.h

@@ -999,6 +999,7 @@ public:
 	      COND *conds, uint og_num, ORDER *order, ORDER *group,
 	      Item *having, ORDER *proc_param, SELECT_LEX *select,
 	      SELECT_LEX_UNIT *unit);
+  bool prepare_stage2();
   int optimize();
   int reinit();
   int init_execution();