update init script and systemd service file

update init script and systemd service file

init script: depend on $remote_fs instead of $local_fs. Reason: /usr can
be mounted a remote filesystem. Also depend on mpd (instead of $mpd,
which doesn't seem to exist.. at least on my system)

Also provide a default config (which should be installed as
/etc/default/ympd)

This commit makes the systemd unit file as well as the init script load
values from this configuration file so the user can change ympd settings
easily when running it as a service.