MFC r222806:

MFC r222806:
  Make a behaviour of the libalias based in-kernel NAT a bit closer to
  how natd(8) does work. natd(8) drops packets only when libalias returns
  PKT_ALIAS_IGNORED and "deny_incoming" option is set, but ipfw_nat
  always did drop packets that were not aliased, even if they should
  not be aliased and just are going through.

  PR:		kern/122109, kern/129093, kern/157379
  Submitted by:	Alexander V. Chernikov (previous version)

git-svn-id: svn://svn.freebsd.org/base/stable/8@223835 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f