Add the MNT_* flags to be used e.g. with Getfsstat on FreeBSD.

Change-Id: I1afb50f715f447f85b4ece3d3fead8b9ce79c4f6
Reviewed-on: https://go-review.googlesource.com/70250Reviewed-by: Matt Layher <mdlayher@gmail.com>
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>