Allow graceful fast exit of cron.php / adhoc_task.php via SIGTERM or SIGINT

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:

3) Run the standalone ad hoc task processor:

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:

5) Repeat the above, but press Control C twice, the second time it will do a hard exit.

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.