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.

  # ipfw nat64clat CLAT create clat_prefix SRC_PFX plat_prefix DST_PFX
  # ipfw add nat64clat CLAT ip4 from IPv4_PFX to any out
  # ipfw add nat64clat CLAT ip6 from DST_PFX to SRC_PFX in

Obtained from:	Yandex LLC
Submitted by:	Boris N. Lytochkin
Relnotes:	yes
Sponsored by:	Yandex LLC