ipfw3nat: kernel NAT without libalias

ipfw3nat: kernel NAT without libalias

More than just one address can be configured in each NAT
configuraton and each coming packet will be translated to
one of these addresses randomly.

e.g.
    ipfw3 nat 1 config ip 192.168.1.1 192.168.1.2

A schedule will automatically purge the expired translations.
and its interval can be configure via below sysctl variables.
    net.inet.ip.fw3_nat.udp_timeout
    net.inet.ip.fw3_nat.tcp_timeout
    net.inet.ip.fw3_nat.icmp_timeout