systemd: add user unit

The user unit omits the "ProtectKernelModules" setting which fails
with modular kernels:

 Failed at step CAPABILITIES spawning /usr/bin/mpd: Operation not permitted

It is unfortunate that systemd (version 232) is unable to reduce its
own capabilities, because this requires us to split system and user
units.

 https://bugs.musicpd.org/view.php?id=4608

systemd/user/mpd.service.in

@@ -0,0 +1,29 @@
+[Unit]
+Description=Music Player Daemon
+After=network.target sound.target
+
+[Service]
+ExecStart=@prefix@/bin/mpd --no-daemon
+
+# allow MPD to use real-time priority 50
+LimitRTPRIO=50
+LimitRTTIME=infinity
+
+# disallow writing to /usr, /bin, /sbin, ...
+ProtectSystem=yes
+
+# more paranoid security settings
+NoNewPrivileges=yes
+ProtectKernelTunables=yes
+ProtectControlGroups=yes
+# AF_NETLINK is required by libsmbclient, or it will exit() .. *sigh*
+RestrictAddressFamilies=AF_INET AF_INET6 AF_UNIX AF_NETLINK
+RestrictNamespaces=yes
+
+# Note that "ProtectKernelModules=yes" is missing in the user unit
+# because systemd 232 is unable to reduce its own capabilities
+# ("Failed at step CAPABILITIES spawning /usr/bin/mpd: Operation not
+# permitted")
+
+[Install]
+WantedBy=default.target