Don't return error on normal shutdown

Previously we've always returned error code from main() even in case of
regular expected shutdown. Let's not confuse it with actual error
shutdown and return 0 by default.

Change-Id: I7fe0d3e052953d5b87ce65649d88d83476fee3c0

ggsn/ggsn.c

@@ -1158,5 +1158,5 @@ int main(int argc, char **argv)
 	llist_for_each_entry(ggsn, &g_ggsn_list, list)
 		ggsn_stop(ggsn);
 
-	return 1;
+	return 0;
 }