natd(8) will work with an unconfigured interface and effectively not do
anything until the interface is assigned an address. This fixes
ipfw_nat to do the same by using an IP of INADDR_ANY instead of
aborting the nat setup if the requested interface is not yet configured.

Differential Revision: https://reviews.freebsd.org/D1539
Reviewed by: melifaro, glebius, gnn
MFC after: 1 week