MFC r303012:

MFC r303012:
  Add ipfw_nptv6 module that implements Network Prefix Translation for IPv6
  as defined in RFC 6296. The module works together with ipfw(4) and
  implemented as its external action module. When it is loaded, it registers
  as eaction and can be used in rules. The usage pattern is similar to
  ipfw_nat(4). All matched by rule traffic goes to the NPT module.

  Reviewed by:	hrs
  Obtained from:	Yandex LLC
  Relnotes:	yes
  Sponsored by:	Yandex LLC
  Differential Revision:	https://reviews.freebsd.org/D6420

MFC r304049:
  Add `stats reset` command implementation to NPTv6 module
  to be able reset statistics counters.

  Obtained from:	Yandex LLC
  Sponsored by:	Yandex LLC

MFC r304076:
  Make statistics nat64lsn, nat64stl an nptv6 output netstat-like:
  "@value @description" and fix build due to -Wformat errors.

MFC r314507:
  Fix NPTv6 rule counters when one_pass is not enabled.

  Consider the rule matching when both @done and @retval values
  returned from ipfw_run_eaction() are zero. And modify ipfw_nptv6()
  to return IP_FW_DENY and @done=0 when addresses do not match.

  Obtained from:	Yandex LLC
  Sponsored by:	Yandex LLC