Reference: https://www.illumos.org/man/2/statvfs

Change-Id: If7af43da35b3204f5069a48eb08426a145544008
Reviewed-on: https://go-review.googlesource.com/43490Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>