- Add unsigned flag to user_var_entry, used when 'type' is INT_RESULT
- Propagate unsigned flag from the query executed by Item_single_row_subselect


mysql-test/r/user_var.result:
  Update test results
mysql-test/t/user_var.test:
  Add test case
sql/item_func.cc:
  Add unsigned_flag to user_var_entry. Used when 'type' is INT_RESULT
  Pass unsigned_flag to 'update_hash' if type is INT_RESULT
sql/item_func.h:
  Removed unused variable save_buff
  Add parameter unsigned_arg to 'update_hash'
sql/item_subselect.cc:
  Propagate unsigned_flag to Item_singlerow_subselect from the items in the select to the cached items.
sql/sql_class.h:
  Add unsigned_flag to user_var_entry. Used when 'type' is INT_RESULT