names with ident. tables fr. diff. schemata": revise all uses of
Item_field and make them prepared-statements friendly when necessary.


mysql-test/r/ps.result:
  Test results fixed: the test case for Bug#6050
mysql-test/r/ps_1general.result:
  Test results fixed: in prepared statements we expand '*' to a list
  of fully qualified fields (db.table.column).
mysql-test/t/ps.test:
  A test for Bug#6050 "EXECUTE stmt reports ambiguous fieldnames with 
  ident. tables fr. diff. schemata"
sql/item.cc:
  Revise all Item_field constructors: we need to make sure that no Item_field
  object points to unaccessible memory in prepared statements.
sql/item.h:
  Revise all Item_field constructors: we need to make sure that no Item_field
  object points to unaccessible memory in prepared statements.
sql/sql_base.cc:
  Item_field use changed to be prepared statements friendly.
sql/sql_class.h:
  New check of Item_arena state.
sql/sql_union.cc:
  Fixing the problem with name resolving in UNION and prepared statements:
  In case of SELECT a, b, c FROM t1 UNION SELECT a, b, c FROM t2 the list of
  selected items is represented as a List<Item_field>, where each 
  Item_field points to a field of temporary table.
  But the temporary table is created anew on each execution of the prepared 
  statement. So on each subsequent execution we should reset Item_field
  items to point to fields from freshly-created temporary table.
sql/table.h:
  Comment TABLE member.