Significantly update tcp_lro for mostly two things:
  1) introduce basic support for IPv6 without extension headers.
  2) try hard to also get the incremental checksum updates right,
     especially also in the IPv4 case for the IP and TCP header.

  Move variables around for better locality, factor things out into
  functions, allow checksum updates to be compiled out, ...

  Leave a few comments on further things to look at in the future,
  though that is not the full list.

  Update drivers with appropriate #includes as needed for IPv6 data
  type in LRO.

  Sponsored by:	The FreeBSD Foundation
  Sponsored by:	iXsystems

Reviewed by:	gnn (as part of the whole)
MFC After:	3 days