vdr-addon: iptv option for number of devices

The iptv-addon has support for more than a single channel. The number of available devices can be set using the -d (or --devices) option (see iptv.c in the sources of the plugin).

I've added an option to the config menu of the vdr-addon to set the number of devices to 1..4 (default 1). The upper limit of 4 is choosen randomly - I myself am happy with 2 devices (background recording while watching another channel).