Bring in some last changes in NAT64 implementation

[ae]

Summary

o Modify ipfw(8) to be able set any prefix6 not just Well-Known,
  and also show configured prefix6;
o relocate some definitions and macros into proper place;
o convert nat64_debug and nat64_allow_private variables to be
  VNET-compatible;
o add struct nat64_config that keeps generic configuration needed
  to NAT64 code;
o add nat64_check_prefix6() function to check validness of specified
  by user IPv6 prefix according to RFC6052;
o use nat64_check_private_ip4() and nat64_embed_ip4() functions
  instead of nat64_get_ip4() and nat64_set_ip4() macros. This allows
  to use any configured IPv6 prefixes that are allowed by RFC6052;
o introduce NAT64_WKPFX flag, that is set when IPv6 prefix is
  Well-Known IPv6 prefix. It is used to reduce overhead to check this;
o modify nat64lsn_cfg and nat64stl_cfg structures to use nat64_config
  structure. And respectivelly modify the rest of code;
o remove now unused ro argument from nat64_output() function.

[ae]

Committed in r333403.