Allow use of interface name in rules file. This way if you have a dynamic

Allow use of interface name in rules file.  This way if you have a dynamic
IP address you don't have to edit the rules file every time you connect.

Example of usage:

coyote% cat /etc/nat.rules
map ppp0 10.0.0.0/8 -> ppp0/32 portmap tcp/udp 10000:20000

ppp0 is automagically transformed into the IP address of the ppp0 interface.