On BSD variants, for some historical reason, the data format used by raw
IP socket may differ from the IPv4 wire format. This change clarifies
that input and output of Header type must conform to the raw IP socket
format.

Change-Id: I6ca363f7ea9a3d7645ee81b588785204dee00cba
Reviewed-on: https://go-review.googlesource.com/128215Reviewed-by: Ian Lance Taylor <iant@golang.org>