Release cron locks on abnormal script termination (if possible)

If a cron job is killed while running a scheduled task, it leaves an open lock on the task. In the case of the database lock factory, and probably others, it's a whole day before this lock is released, so the task is effectively disabled for a day.

Where the PCNTL functions are enabled in PHP, it would be possible to register a handler for SIGTERM, SIGINT and SIGKILL that would mark the task as failed before exiting and prevent this problem.