When enabling DLNA, also make sure to enable the plugin that
we use to export the media to the TV or set-top-box, otherwise
rygel won't export anything.
https://bugzilla.gnome.org/show_bug.cgi?id=708055
Ensure Rygel is started when the Media Sharing option is enabled and
stopped when it is disabled. This also adds and removes Rygel from the
autostart directory as appropriate.
https://bugzilla.gnome.org/show_bug.cgi?id=692702