MFC r345262:

MFC r345262:
  Modify struct nat64_config.

  Add second IPv6 prefix to generic config structure and rename another
  fields to conform to RFC6877. Now it contains two prefixes and length:
  PLAT is provider-side translator that translates N:1 global IPv6 addresses
  to global IPv4 addresses. CLAT is customer-side translator (XLAT) that
  algorithmically translates 1:1 IPv4 addresses to global IPv6 addresses.
  Use PLAT prefix in stateless (nat64stl) and stateful (nat64lsn)
  translators.

  Modify nat64_extract_ip4() and nat64_embed_ip4() functions to accept
  prefix length and use plat_plen to specify prefix length.

  Retire net.inet.ip.fw.nat64_allow_private sysctl variable.
  Add NAT64_ALLOW_PRIVATE flag and use "allow_private" config option to
  configure this ability separately for each NAT64 instance.

  Obtained from:	Yandex LLC
  Sponsored by:	Yandex LLC