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 FreeBSD's
sys/types.h header. However, the parameter and return types are changed
to match the existing implementations of these functions.

Because FreeBSD dynamically allocates major/minor device numbers through
devfs at runtime, testing the conversion macros against well-known
device numbers is not possible.

Updates golang/go#8106

Change-Id: I9874e63f388cfc5c0d5cc47d4d0f6ccc489b28f2
Reviewed-on: https://go-review.googlesource.com/61631Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>