When cancelling a run, engine processes survive

[akerbos]

When we cancel a run with CTRL+C, engine processes that were started are not (always) killed. They even survive termination of the original ltx2any process!

Investigate and resolve.

[akerbos]

Possible solution: store PIDs of all running child processes and skill them explicitly.

http://stackoverflow.com/questions/32965251/how-can-i-kill-process-which-has-spawned-a-child-using-system

Getting the PID is easy: f = IO::popen(...); f.pid
Killing as well: Process.kill("TERM", pid)

[akerbos]

Other possibilty: set up a signal handler.

http://stackoverflow.com/a/14636093/539599

Do we still want to deal with CTRL+C differently is daemon mode is running?
Could use this to completely overhaul how signals are processed -- which should probably happen, anyway.