- a fix for Bug#11458 "Prepared statement with subselects return random

data": remove the fix for another bug (8807) that
added OUTER_REF_TABLE_BIT to all subqueries that used a placeholder
to prevent their evaluation at prepare. As this bit hanged in 
Item_subselect::used_tables_cache for ever, a constant subquery with
a placeholder was never evaluated as such, which caused wrong 
choice of the execution plan for the statement.
- to fix Bug#8807 backport a better fix from 5.0
- post-review fixes.


mysql-test/r/ps.result:
  Bug#11458: test results fixed
mysql-test/t/ps.test:
  - add a test case for Bug#11458 "Prepared statement with subselects return 
  random data"
sql/item.cc:
  - remove unnecessary Item_param::fix_fields
  - fix Item_param::set_null to set item_type accordingly (safety:
    Item_param should behave like a basic constant).
sql/item.h:
  Remove Item_param::fix_fields
sql/item_subselect.h:
  Remove no more existing friend.
sql/mysql_priv.h:
  Add UNCACHEABLE_PREPARE to mark all subqueries as uncacheable if
  in statement prepare (backport from 5.0).
sql/sql_lex.h:
  Comment fixed.
sql/sql_parse.cc:
  If in statement prepare, mark all subqueries as uncacheable (backport
  from 5.0)
sql/sql_prepare.cc:
  Switch off the uncacheable flag from all subqueries after statement
  prepare is done (backport from 5.0)

sql/sql_lex.h

@@ -257,6 +257,7 @@ public:
       UNCACHEABLE_RAND
       UNCACHEABLE_SIDEEFFECT
       UNCACHEABLE_EXPLAIN
+      UNCACHEABLE_PREPARE
   */
   uint8 uncacheable;
   enum sub_select_type linkage;