On BSD variaints, for some historical reason, the data format used by raw
ICMP socket may differ from the IPv4 wire format and the format used by
raw IP socket. This change clarifies that input of ParseIPv4Header must
conform to the raw ICMP socket format.

Change-Id: I7288eccaaae0662d0437794098c8f7fc4a55d81e
Reviewed-on: https://go-review.googlesource.com/128216Reviewed-by: Ian Lance Taylor <iant@golang.org>