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.

• Boot my laptop which NATs over wlan0 to allow VM guests to access the outside world. ipfw is configured before wpa_supplicant/dhclient have had a chance to configure wlan0. With natd this works fine. With ipfw_nat, the nat rules fail to install causing ipfw to exit and not install the full rule set, leaving me with a default deny rule.