Try to allow children to shut down gracefully

Since signals will no longer be handled once foreman goes into
`terminate_gracefully`, default signal handlers are restored so as
not to cause it to get stuck in an unTERMable state.

This necessitates not using the process group for signalling
except as a last resort, as foreman itself will receive the signals
it sends. This splits `killall` into two methods, one which
signals only processes foreman itself has started, and one which
signals all processes in the process group to try to clean up
more aggressively, and then reworks `terminate_gracefully` to use
them.