Support daemon supervision by upstart or systemd

Both upstart and systemd provide a way for daemons to
be supervised, as well as a mechanism for them to
signal their readyness status.

This patch provides compatibility with this functionality while
not interfering with other methods.

With this, it will be possible to use `expect stop` with upstart
and `Type=notify` with systemd.

A more detailed explanation of the mechanism can be found here:
http://spootnik.org/entries/2014/11/09_pid-tracking-in-modern-init-systems.html

src/debug.c

@@ -857,7 +857,7 @@ void sigsegvHandler(int sig, siginfo_t *info, void *secret) {
 "  Suspect RAM error? Use redis-server --test-memory to verify it.\n\n"
 );
     /* free(messages); Don't call free() with possibly corrupted memory. */
-    if (server.daemonize) unlink(server.pidfile);
+    if (server.daemonize && server.supervised == 0) unlink(server.pidfile);
 
     /* Make sure we exit with the right signal at the end. So for instance
      * the core will be dumped if enabled. */