These types and constants are taken from hdreg.h, and will enable
querying and manipulation of block devices on Linux.

Fixes golang/go#24971

Change-Id: Iafe8f81ba7ab9f1e127743dab6a9fb1912874a5c
Reviewed-on: https://go-review.googlesource.com/108455
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>