MFC r339542:

MFC r339542:
  Retire IPFIREWALL_NAT64_DIRECT_OUTPUT kernel option. And add ability
  to switch the output method in run-time. Also document some sysctl
  variables that can by changed for NAT64 module.

  NAT64 had compile time option IPFIREWALL_NAT64_DIRECT_OUTPUT to use
  if_output directly from nat64 module. By default is used netisr based
  output method. Now both methods can be used, but they require different
  handling by rules.

  Obtained from:	Yandex LLC
  Sponsored by:	Yandex LLC
  Differential Revision:	https://reviews.freebsd.org/D16647