Open a new process group before setting up signal handler

When cleaning up after a caught signal, quark forwards the signal to all
processes in the process group with `kill(0, ...)`. If we do not open up a new
process group in the parent process, quarks parent will be sent a SIG... too,
resulting it to shut down (especially considering that the parent process might
run as root).

As a result, if we set up the service with djb's excellent daemontools,
`svc -d quark` will terminate the svscan-process and tear all other services
down with it.

See also <https://cr.yp.to/daemontools/faq/create.html#pgrphack>.

main.c

@@ -240,6 +240,9 @@ main(int argc, char *argv[])
 		    "Entry not found");
 	}
 
+	/* Open a new process group */
+	setpgid(0,0);
+
 	handlesignals(sigcleanup);
 
 	/* bind socket */