Introduce a (for now copied stripped down) in6_cksum_pseudo()
function. We should be able to use this from in6_cksum() but
we should also ponder possible MD specific improvements.
It takes an extra csum argument to allow for easy checks as
will be done by the upper layer protocol input paths.
Sponsored by: The FreeBSD Foundation
Sponsored by: iXsystems
Reviewed by: gnn (as part of the whole)
MFC After: 3 days