FreeBSD 10 kernel has changed its incoming IPv4 packet handling
to stop trimming some IPv4 header fields on raw socket IO. This CL
just adapts package's IPv4 header representation to look the same
even on FreeBSD 10 kernel.

For further information:
http://svnweb.freebsd.org/base/head/?view=log&pathrev=226105
http://svnweb.freebsd.org/base/head/?view=log&pathrev=241913
http://svnweb.freebsd.org/base/head/?view=log&pathrev=241923

LGTM=iant
R=golang-codereviews, gobot, dave, iant
CC=golang-codereviews
https://golang.org/cl/53030043