Add ability to send arbitrary UNIX signals to processes:

* Extend the Process class to handle non-killing signals
* Expose this via the XML-RPC API
* Add supervisorctl `signal` command

This is an update of PR #228 and hopefully moves the ball forward on #179, #53, etc.

Cc: @squeed, @weebly, @mnaberez, @sontek
We have this in [datatypes.signal_number](https://github.com/Supervisor/supervisor/blob/3.1.1/supervisor/datatypes.py#L392-L404).