systemd-userdbd: add option to enable

This service is shipped as a part of upstream systemd but not currently
incldued in NixOS. This change adds a configuration option which may
be used to enable it

nixos/modules/module-list.nix

@@ -1286,6 +1286,7 @@
   ./system/boot/systemd/shutdown.nix
   ./system/boot/systemd/tmpfiles.nix
   ./system/boot/systemd/user.nix
+  ./system/boot/systemd/userdbd.nix
   ./system/boot/timesyncd.nix
   ./system/boot/tmp.nix
   ./system/boot/uvesafb.nix

nixos/modules/system/boot/systemd/userdbd.nix

@@ -0,0 +1,26 @@
+{ config, lib, pkgs, utils, ... }:
+with lib;
+let
+  cfg = config.systemd.userdbd;
+  systemd = config.systemd.package;
+in
+{
+  options = {
+    systemd.userdbd.enable = mkOption {
+      default = false;
+      type = types.bool;
+      description = ''
+        Whether to enable the systemd-userdbd user/group DB lookup service
+      '';
+    };
+  };
+
+  config = mkIf cfg.enable {
+    systemd.additionalUpstreamSystemUnits = [
+      "systemd-userdbd.socket"
+      "systemd-userdbd.service"
+    ];
+
+    systemd.sockets.systemd-userdbd.wantedBy =  [ "sockets.target" ];
+  };
+}