vss: Avoid large FEC parameters for the DCCP transport.

vss: Avoid large FEC parameters for the DCCP transport.

Now that for DCCP streams the audio file header is sent only once at client
connect time, we can go one step further and send the header as its own FEC
group. Then all subsequent FEC groups contain data slices only, hence the
maximal required size for a FEC group reduces from

	header_size + largest_chunk_size

to

	max(header_size, largest_chunk_size)

This patch introduces a new helper function need_data_slices() which returns
false only at the beginning of a DCCP stream. In this case FEC group 0 consists
of the header only and an arbitrary time interval of 200ms is used for this
group.