4617: Don't handle SIGTERM for glideinFrontendElement children

As with issue 4436, this will avoid the race wtih async exceptions
(receiving a second signal while handling the first). We unregister
the signal handler before forking, and re-register the parent only
as soon as we can.
4617: Move process group id handling into common library
4617: Let frontend become a process group leader

With this change, the frontend can be the process group leader
for any children it forks -- useful for controlled stopping
of the frontend.
4617: Change stopFrontend to use process groups

This allows the stopFrontend code to just send a TERM
to the process group leader and let unix propagate the signal.