Designate one process to maintain the PID file. Coordinator does that

Designate one process to maintain the PID file. Coordinator does that
when Squid runs in SMP mode. The only kid does it when running "mono".
The master process does it when not daemonizing. This change allows SMP
Squid to work with a single PID file.

Broadcast management signals from Coordinator to kids. This change
makes both "squid -k ..." and "kill `cat <PID file>`" work for SMP Squid.