Allow graceful fast exit of cron via sigterm

1) Install this plugin just to make testing easier: https://github.com/catalyst/moodle-tool_testtasks   2) Queue up 5 ad hoc tasks that each take 5 seconds to run: php admin/tool/testtasks/cli/queue_adhoc_tasks.php -n= 5 -d= 5   3) Run the standalone ad hoc task processor: php admin/tool/task/cli/adhoc_task.php --execute   4) Watch it for a little while, then press Control C, instead of immediately exiting it should gracefully finish what it was doing, and then exit instead of picking up it's next task: == Exiting gracefully, please wait ... ==   5) Repeat the above, but press Control C twice, the second time it will do a hard exit. == Exiting right NOW ==    6)  Rinse and repeat with admin/cli/cron.php but you need to carefully kill it during both the scheduled tasks and the ad hoc tasks.  7) The tests above capture SIGINT, for completeness also re-test but instead of killing via Ctrl+C, open another terminal and kill is using: kill [pid] This should also result in the 'Exiting gracefully' message.