audiod: Make default filters work also for udp streams.

audiod: Make default filters work also for udp streams.

Currently one has to use --no_default_filters and must specify fecdec as the
first filter for UDP streams, a fact that is not documented anywhere. Make decoding
of udp streams work if no filters were given by changing init_default_filters() to
add fecdec automatically if the udp receiver is used.