fix wrong return code

In case we are a daemon, we do not need to daemonize again.
On the other hand everything is fine and we also do not need
to bail out with an error.

The daemonize template at [1] does the same.

[1] http://www.itp.uzh.ch/~dpotter/howto/daemonize

Change-Id: Ia4dcf7344bd65934faa3d7d46563f6e0532c232e

src/application.c

@@ -124,7 +124,7 @@ int osmo_daemonize(void)
 
 	/* Check if parent PID == init, in which case we are already a daemon */
 	if (getppid() == 1)
-		return -EEXIST;
+		return 0;
 
 	/* Fork from the parent process */
 	pid = fork();