Unlocks the database. Any session may steal a database lock; no checking of any
sort is done.
The possible responses are:
200 (CODE_OK) The lock has been removed.
432 (CODE_GNATS_NOT_LOCKED) The database was not locked.
440 (CODE_CMD_ERROR) One or more arguments were supplied to the command.
6xx (internal error) The database lock could not be removed, usually because of
permissions or other filesystem-related issues.