unix: add Major, Minor and Mkdev functions on Darwin

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 Darwin's
sys/types.h header. However, the parameter and return types are changed
to match the respective Linux implementation of these functions.

Test the conversion macros with some well-known static device numbers
for devices which should be present on any Darwin system.

Updates golang/go#8106

Change-Id: I1862be64684cc1b5a53e15a883819571e368cb2b
Reviewed-on: https://go-review.googlesource.com/60610
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/dev_darwin.go

@@ -0,0 +1,24 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Functions to access/create device major and minor numbers matching the
+// encoding used in Darwin's sys/types.h header.
+
+package unix
+
+// Major returns the major component of a Darwin device number.
+func Major(dev uint64) uint32 {
+	return uint32((dev >> 24) & 0xff)
+}
+
+// Minor returns the minor component of a Darwin device number.
+func Minor(dev uint64) uint32 {
+	return uint32(dev & 0xffffff)
+}
+
+// Mkdev returns a Darwin device number generated from the given major and minor
+// components.
+func Mkdev(major, minor uint32) uint64 {
+	return uint64((major << 24) | minor)
+}