Fixed context in error mesages


mysql-test/r/subselect.result:
  tests of references and error message
mysql-test/t/subselect.test:
  tests of references and error message
sql/item.cc:
  Fixed layout
  Resolving field names of outer select
sql/item_subselect.cc:
  saving/restoring context for error messages
sql/mysql_priv.h:
  changed function interface to allow resolving field names inside Item_ref::fix_fields
sql/sql_base.cc:
  changed function interface to allow resolving field names inside Item_ref::fix_fields
sql/sql_lex.h:
  allow access to thd field