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
diff --git a/ggsn/ggsn.c b/ggsn/ggsn.c
index 39695b9..dd723e9 100644
--- a/ggsn/ggsn.c
+++ b/ggsn/ggsn.c
@@ -1158,5 +1158,5 @@
 	llist_for_each_entry(ggsn, &g_ggsn_list, list)
 		ggsn_stop(ggsn);
 
-	return 1;
+	return 0;
 }