nulldevice release

If dbus2vdr is started with the parameter "--nulldevice", it creates an output device similar to the famous dummydevice-plugin. Using the call "device.RequestPrimary" the active primary device can be switched to suspend in- and output of live-tv. The calls "device.List", "device.GetPrimary" and "device.GetNullDevice" helps to develop tools for switching the primary device.