unix: add Major, Minor and Mkdev functions on FreeBSD

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>

unix/dev_freebsd.go

+// 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 FreeBSD's sys/types.h header.
+//
+// The information below is extracted and adapted from sys/types.h:
+//
+// Minor gives a cookie instead of an index since in order to avoid changing the
+// meanings of bits 0-15 or wasting time and space shifting bits 16-31 for
+// devices that don't use them.
+
+package unix
+
+// Major returns the major component of a FreeBSD device number.
+func Major(dev uint64) uint32 {
+	return uint32((dev >> 8) & 0xff)
+}
+
+// Minor returns the minor component of a FreeBSD device number.
+func Minor(dev uint64) uint32 {
+	return uint32(dev & 0xffff00ff)
+}
+
+// Mkdev returns a FreeBSD device number generated from the given major and
+// minor components.
+func Mkdev(major, minor uint32) uint64 {
+	return uint64((major << 8) | minor)
+}