dhcp6c subscribe to ppp ifindex

when the peer bounces ppp, s6 will restart the ppp process but not
restart the dependent services (because the service isn't considered
to have gone down)

so the dependent services need to notice when the outputs from ppp
have changed

modules/dhcp6c/acquire-delegated-prefix.fnl

@@ -25,6 +25,7 @@
 (fn run []
   (let [[state-directory lan-device] arg
         dir (svc.open state-directory)]
+    (update-prefixes lan-device [] (or (dir:output "prefix") []) system)
     (accumulate [addresses []
                  v (dir:events)]
       (update-prefixes lan-device addresses (or (v:output "prefix") []) system))))