add "waitup", s6 readiness helper for network interfaces

run e.g. "waitup wlan0 10" to wait until wlan0 is operationally
up and running, and then send a newline to file descriptor 10

pkgs/netlink-lua/default.nix

@@ -0,0 +1,23 @@
+{ lua, lib, fetchpatch, fetchFromGitHub, libmnl }:
+let pname = "netlink";
+in lua.pkgs.buildLuaPackage {
+  inherit pname;
+  version = "0.1.1-1";
+
+  buildInputs = [ libmnl ];
+
+  src = fetchFromGitHub {
+    repo = "lua-netlink";
+    owner = "chris2511";
+    rev = "v0.1.1";
+    hash = "sha256:1833naskl4p7rz5kk0byfgngvw1mvf6cnz64sr3ny7i202wv7s52";
+  };
+
+  buildPhase = "$CC -shared -l mnl -l lua -o netlink.so src/*.c";
+
+  installPhase = ''
+    mkdir -p "$out/lib/lua/${lua.luaversion}"
+    cp  netlink.so "$out/lib/lua/${lua.luaversion}/"
+  '';
+
+}