UDP catchall

[Julian Elischer]

It's possible using ipfw to mostly implement this, and with an upcoming change, possible 
to completely implement this.

the "uid" function of ipfw can act as a "does there exist a socket to which this packet would go?" test.
and a variant of it called "for_me" that I am adding (we use it at work) does this even better.

so, basically,

yyy:   skipto xxx ip from any to-me
yyy+1: fwd 127.0.0.1,1234
xxx: