mysql-test/r/sp.result:
  Test cases for user variables (@-vars) as OUT and INOUT parameters.
mysql-test/t/sp.test:
  Test cases for user variables (@-vars) as OUT and INOUT parameters.
sql/item_func.h:
  We need to be able to detect Item_func_get_user_vars when used as arguments to SPs.
sql/sp_head.cc:
  Handle user variables (@-vars) as OUT and INOUT parameters.
  But we still just ignore it, if it's anything else.