supervise-daemon: multiple fixes

- Harden against dying by handling all signals that would terminate the
program and adding --reexec support
- factor the supervisor into its own function
- fix test for whether we are already running

src/rc/rc-misc.c

@@ -217,6 +217,18 @@ signal_setup(int sig, void (*handler)(int))
 	return sigaction(sig, &sa, NULL);
 }
 
+int
+signal_setup_restart(int sig, void (*handler)(int))
+{
+	struct sigaction sa;
+
+	memset(&sa, 0, sizeof (sa));
+	sigemptyset(&sa.sa_mask);
+	sa.sa_handler = handler;
+	sa.sa_flags = SA_RESTART;
+	return sigaction(sig, &sa, NULL);
+}
+
 int
 svc_lock(const char *applet)
 {