systemd socket activation

Using systemd, you can launch MPD on demand when the first client attempts to connect.

MPD comes with two systemd unit files: a "service" unit and a "socket" unit. These will only be installed when MPD was configured with --with-systemdsystemunitdir=/lib/systemd.

To enable socket activation, type:

systemctl enable mpd.socket
systemctl start mpd.socket

In this configuration, MPD will ignore the bind_to_address and port settings.