add rport(4) for p2p l3 connectivity between route domains.

add rport(4) for p2p l3 connectivity between route domains.

you can basically plug rdomains together and route between them
over rport interfaces. people keep asking me if this is so you can
leak routes between rdomains, and the answer is yes.

this is like pair(4) but cheaper because it avoids all the mucking
around with putting an ethernet header on the mbuf just to take it
off again later, and is more efficient with address space because
it's a p2p ip interface.

it has a small tweak from mvs@
ok denis@ claudio@