bpf will see inner and outer headers or just inner or outer
headers for incoming and outgoing IPsec packets.
This is useful in bpf to not have over long lines for debugging
or selcting packets based on the inner headers.
It also properly defines the behavior of what the firewalls see.
Last but not least it gives you if_enc(4) for IPv6 as well.
[ As some auxiliary state was not available in the later
input path we save it in the tdbi. That way tcpdump can give a
consistent view of either of (authentic,confidential) for both
before and after states. ]
Discussed with: thompsa (2007-04-25, basic idea of unifying paths)
Reviewed by: thompsa, gnn
19ad9831df