Update User_level_lock::thread_id on acquiring an existing lock,
and reset it on lock release.


mysql-test/r/func_misc.result:
  Add result for bug#16501.
mysql-test/t/func_misc.test:
  Add test case for bug#16501.
sql/item_func.cc:
  Update User_level_lock::thread_id on acquiring an existing lock,
  and reset it on lock release (for safety).