This CL tweaks syscall.InterfaceMessage to support FreeBSD 10 and
prior to 10.

See http://svnweb.freebsd.org/base?view=revision&revision=254804.

Fixes #7193.

LGTM=iant
R=golang-codereviews, rsc, minux.ma, gobot, iant
CC=golang-codereviews
https://golang.org/cl/56980043