Reliable shutdown/cleanup of drunc-controller, etc. processes would be very beneficial

[bieryAtFnal]

... when the drunc-unified-shell exits.

At the moment, we see lingering processes stay around for 2-8 seconds after we cleanly exit the drunc-unified-shell.

This Issue is requesting that the drunc-unified-shell ensure that all of its child processes are gone when it exits, both when the shutdown of the DAQ session is graceful and when it is not.

This will allow users of drunc to start a new session immediately after a previous one finishes, without worrying about leftover processes contaminating the second session.