libthread: call setpgrp in programs that will background

This fixes the 'run stats from rc; exit rc; stats dies' problem.
It's unclear whether this is the right fix or whether rc should
be starting all its interactive commands in their own process
groups. But at least it does fix stats dying.

src/libthread/daemonize.c

@@ -101,6 +101,13 @@ _threadsetupdaemonize(void)
 
 	sigpid = 1;
 
+	/*
+	 * We've been told this program is likely to background itself.
+	 * Put it in its own process group so that we don't get a SIGHUP
+	 * when the parent exits.
+	 */
+	setpgrp();
+
 	if(pipe(p) < 0)
 		sysfatal("passer pipe: %r");