ipfw3: lockless in-kernel NAT

ipfw3: lockless in-kernel NAT

The libalias is used in kernel space for in-kernel NAT, and its alias_link
entries are stored with LIST. so all the packets which need to be NAT will scan
against the LIST and trying to find the matched alias_link. by seperating the
libalias into context of different CPUs, the lock can be removed. and due to the
nature of NAT, the outgoing and incoming packets are possible to be handled by
different CPUs, to ensure the returning packet can be translated properly, the
newly created alias_link is required to be duplicated and inserted into contexts
of both CPUs.

e.g.
ipfw3 nat 1 config if em0
ipfw3 nat 1 all via em0
ipfw3 nat 1 show state