add mdevd as module

following the upstream example, it republishes uevent messages
using multicast group 4 instead of group 2 as used by udev.

modules/all-modules.nix

@@ -19,6 +19,7 @@
    ./kernel
    ./outputs/kexecboot.nix
    ./mount
+   ./mdevd.nix
    ./network
    ./ntp
    ./outputs.nix

modules/mdevd.nix

@@ -0,0 +1,17 @@
+{ config, pkgs, lib, ...} :
+let inherit (pkgs.liminix.services) oneshot longrun bundle target;
+in {
+  config = {
+    services = rec {
+      mdevd = longrun {
+        name = "mdevd";
+        notification-fd = 3;
+        run = "${pkgs.mdevd}/bin/mdevd -D 3 -b 200000 -O4";
+      };
+      mdevd-coldplug = oneshot {
+        name ="mdev-coldplug";
+        up = "${pkgs.mdevd}/bin/mdevd-coldplug";
+      };
+    };
+  };
+}