Add forward error correction code to the udp sender/receiver.

Add forward error correction code to the udp sender/receiver.

This patch adds the first draft of a FEC implementation based
on code by Luigi Rizzo.

On the server side, the FEC encoding is done within the virtual
streaming system which also contains the timing routines for sending
a FEC-encoded audio stream. Senders my request such an encoded stream
by calling vss_add_fec_client() with a fec_client_parms structure
that contains the FEC parameters and a callback function which is used
to actually send the data.

On the receiver side, the new fecdec filter is introduced which must
be used to decode a FEC-encoded stream. As the fec parameters are
contained in the header of each data slice of the encoded stream, no
options to this filter are necessary.

ATM, FEC is only used by the udp sender/receiver, but other protocols
can be easily changed to use FEC as well.

This new code is still experimental, lacks documentation and the fec
parameters can currently only be changed by editing the source code.