Sync ng_nat with recent (r222806) ipfw_nat changes:
Sync ng_nat with recent (r222806) ipfw_nat changes:

  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.

Also add SCTP support: mark response packets to skip firewall processing.

MFC after:	1 month

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