Add Major, Minor and Mkdev functions for converting devices numbers to
their major/minor components and vice versa.

The functions follow the behavior of the macros defined in DragonFlyBSD's
sys/types.h header. However, the parameter and return types are changed
to match the existing implementations of these functions.

Test the conversion macros with some well-known device numbers.

Updates golang/go#8106

Change-Id: I84c128dff3108821caaa75dcec620cf5bdb1f32d
Reviewed-on: https://go-review.googlesource.com/61630Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>