Add NAT64 CLAT implementation as defined in RFC6877.

CLAT is customer-side translator that algorithmically translates 1:1
private IPv4 addresses to global IPv6 addresses, and vice versa.
It is implemented as part of ipfw_nat64 kernel module. When module
is loaded or compiled into the kernel, it registers "nat64clat" external
action. External action named instance can be created using create
command and then used in ipfw rules. The create command accepts two
IPv6 prefixes plat_prefix and clat_prefix. If plat_prefix is ommitted,
IPv6 NAT64 Well-Known prefix 64:ff9b::/96 will be used.

1. ipfw nat64clat CLAT create clat_prefix SRC_PFX plat_prefix DST_PFX
2. ipfw add nat64clat CLAT ip4 from IPv4_PFX to any out
3. ipfw add nat64clat CLAT ip6 from DST_PFX to SRC_PFX in

Obtained from: Yandex LLC
Submitted by: Boris N. Lytochkin
MFC after: 1 month
Relnotes: yes
Sponsored by: Yandex LLC