add service to enable packet forwarding

might be worth looking into adding RA config to this

modules/network/forward.nix

@@ -0,0 +1,21 @@
+{
+  liminix
+, ifwait
+, serviceFns
+, lib
+}:
+{ enableIPv4, enableIPv6 }:
+let
+  inherit (liminix.services) oneshot;
+  ip4 = "/proc/sys/net/ipv4/conf/all/forwarding";
+  ip6 = "/proc/sys/net/ipv6/conf/all/forwarding";
+  opt = lib.optionalString;
+  sysctls = b :
+    ""
+    + opt enableIPv4 "echo ${b} > ${ip4}\n"
+    + opt enableIPv6 "echo ${b} > ${ip6}\n";
+in oneshot {
+  name = "forwarding${opt enableIPv4 "4"}${opt enableIPv6 "6"}";
+  up = sysctls "1";
+  down = sysctls "0";
+}