Add UDP semantics.

Add UDP semantics.

Basically, never retransmit, and whatever we get we immediately pass up to
the application. The only thing we do handle the TCP way is the FIN packet.

This also introduces the use of the aux entry in the header. We know that
older versions of UTCP ignore this field, and also ignore any extra data
in SYN packets, so we can add anything here in a backwards-compatible way.