go.sys/unix: add support for *xattr functions on FreeBSD

Add wrappers that provide Linux-y behavior around the FreeBSD extattr(2)
functions. This allows certain packages, like the fuse package to run
under FreeBSD.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/147850043
5 files changed