Allow ipfw to forward to a destination that is specified by a table.

Allow ipfw to forward to a destination that is specified by a table.
for example:
  fwd tablearg ip from any to table(1)
where table 1 has entries of the form:
1.1.1.0/24 10.2.3.4
208.23.2.0/24 router2

This allows trivial implementation of a secondary routing table implemented
in the firewall layer.

I expect more work (under discussion with Glebius) to follow this to clean
up some of the messy parts of ipfw related to tables.

Reviewed by:	Glebius
MFC after:	1 month