udp: integrate new fec interaction

This patch implements the new UDP-FEC interface in two steps:
 (1) updating the UDP sender analogously to the previous (DCCP sender) patch;
 (2) unifying the struct udp_target by factoring out the fields it has in
     common with 'struct sender_client'.

The open() function is deferred to a subsequent patch. The user can override
the MPS value by supplying a max_slice_bytes value not larger than the MPS.