x/sys/unix: add constants and types from hdreg.h for Linux block device ioctls

[mdlayher]

I'd like to add some constants and types from hdreg.h for the manipulation of block devices under Linux.

I intend to use these in my block package and believe they would be useful in @dswarbrick's smart package as well.

I'll send a CL for this.

[gopherbot]

Change https://golang.org/cl/108455 mentions this issue: unix: add Linux block device ioctl types and constants