For manipulating Linux partitions with the BLKPG ioctl.

Change-Id: I2196038ccfc39ecd6926817c3b310d0837220095
Reviewed-on: https://go-review.googlesource.com/125642
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>