unix: add Major, Minor and Mkdev functions on NetBSD

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 NetBSD'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: I536d6d2622f6fe9be3c1ed3beb266745fe4bfb6e
Reviewed-on: https://go-review.googlesource.com/60970
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_netbsd.go

@@ -0,0 +1,29 @@
+// 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 NetBSD's sys/types.h header.
+
+package unix
+
+// Major returns the major component of a NetBSD device number.
+func Major(dev uint64) uint32 {
+	return uint32((dev & 0x000fff00) >> 8)
+}
+
+// Minor returns the minor component of a NetBSD device number.
+func Minor(dev uint64) uint32 {
+	minor := uint32((dev & 0x000000ff) >> 0)
+	minor |= uint32((dev & 0xfff00000) >> 12)
+	return minor
+}
+
+// Mkdev returns a NetBSD device number generated from the given major and minor
+// components.
+func Mkdev(major, minor uint32) uint64 {
+	dev := uint64((major << 8) & 0x000fff00)
+	dev |= uint64((minor << 12) & 0xfff00000)
+	dev |= uint64((minor << 0) & 0x000000ff)
+	return dev
+}