ympd service: init (#18371)

ympd provides a web ui, it is suitable to be run as a service.
Fixes #17878.

service has no requirements b/c user might be using remote mpd
instance.

nixos/modules/module-list.nix

@@ -114,6 +114,7 @@
   ./services/audio/mpd.nix
   ./services/audio/mopidy.nix
   ./services/audio/squeezelite.nix
+  ./services/audio/ympd.nix
   ./services/backup/almir.nix
   ./services/backup/bacula.nix
   ./services/backup/crashplan.nix

nixos/modules/services/audio/ympd.nix

@@ -0,0 +1,61 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.services.ympd;
+in {
+
+  ###### interface
+
+  options = {
+
+    services.ympd = {
+
+      enable = mkOption {
+        type = types.bool;
+        default = false;
+        description = "Whether to enable ympd, the MPD Web GUI.";
+      };
+
+      webPort = mkOption {
+        type = types.string;
+        default = "8080";
+        description = "The port where ympd's web interface will be available.";
+        example = "ssl://8080:/path/to/ssl-private-key.pem";
+      };
+
+      mpd = {
+        host = mkOption {
+          type = types.string;
+          default = "localhost";
+          description = "The host where MPD is listening.";
+          example = "localhost";
+        };
+
+        port = mkOption {
+          type = types.int;
+          default = config.services.mpd.network.port;
+          description = "The port where MPD is listening.";
+          example = 6600;
+        };
+      };
+
+    };
+
+  };
+
+
+  ###### implementation
+
+  config = mkIf cfg.enable {
+
+    systemd.services.ympd = {
+      description = "Standalone MPD Web GUI written in C";
+      wantedBy = [ "multi-user.target" ];
+      serviceConfig.ExecStart = "${pkgs.ympd}/bin/ympd --host ${cfg.mpd.host} --port ${toString cfg.mpd.port} --webport ${cfg.webPort} --user nobody";
+    };
+
+  };
+
+}