Run IPv6 hop-by-hop options processing in parallel. The ip6_hbhchcheck()

Run IPv6 hop-by-hop options processing in parallel.  The ip6_hbhchcheck()
code is MP safe and moves from ip6_local() to ip6_ours().  If there
are any options, store the chain offset and next protocol in a mbuf
tag.  When dequeuing without tag, it is a regular IPv6 header.  As
mbuf tags degrade performance, use them only if a hop-by-hop header
is present.  Such packets are rare and pf drops them by default.
OK mvs@