firewall: show how to ratelimit icmp6 to 5% of available bandwidth

it's a little klunky as yet, requires setting properties.bandwidth on
the interface service

modules/ppp/common.nix

@@ -115,6 +115,7 @@ let
       ${command}
     '';
     notification-fd = 10;
+#    properties.bandwidth = 3 * 1000 * 1000;
     timeout-up =
       if lcpEcho.failure != null then (10 + lcpEcho.failure * lcpEcho.interval) * 1000 else 60 * 1000;
     inherit dependencies;