Making FLUSH TABLES WITH READ LOCK killable while it's waiting for running commits to finish. Normally this step is not long but it's still nice to be killable
(especially in case of bug like BUG#6732 "FLUSH TABLES WITH READ LOCK + COMMIT makes next FLUSH...LOCK hang forever").


sql/lock.cc:
  making FLUSH TABLES WITH READ LOCK killable while it's waiting for running commits to finish
sql/mysql_priv.h:
  prototype change
sql/sql_parse.cc:
  now it's possible that make_global_read_lock_block_commit fails (killed)