provide mpd user service

The Arch package used to provide this. Now it doesn't.

To use this, you'd just enable and start it as your user.

    $ systemctl --user enable mpd.service
    $ systemctl --user start mpd.service

Existing users can remove the override we used to install, since that is
now redundant.

    # rm -r /etc/systemd/user/mpd.service.d

roles/mpd/files/mpd-service-override.conf

@@ -1,3 +0,0 @@
-[Service]
-ExecStart=
-ExecStart=/usr/bin/firejail /usr/bin/mpd --no-daemon

roles/mpd/files/mpd.service

@@ -0,0 +1,13 @@
+[Unit]
+Description=Music Player Daemon
+After=network.target sound.target
+
+[Service]
+ExecStart=/usr/bin/firejail /usr/bin/mpd --no-daemon
+
+# allow MPD to use real-time priority 50
+LimitRTPRIO=50
+LimitRTTIME=infinity
+
+[Install]
+WantedBy=multi-user.target

roles/mpd/tasks/main.yml

@@ -16,13 +16,8 @@
   tags:
     - firejail
 
-- name: Create mpd systemd unit file directory
-  file: path=/etc/systemd/user/mpd.service.d state=directory
-  tags:
-    - firejail
-
-- name: Push mpd socket unit file
-  copy: src=mpd-service-override.conf dest=/etc/systemd/user/mpd.service.d/override.conf
+- name: Push mpd service file
+  copy: src=mpd.service dest=/etc/systemd/user/mpd.service
   notify:
     - reload systemd config
   tags: