archive/tar: populate Devmajor and Devminor in FileInfoHeader on Darwin

Extract device major/minor number on Darwin and set Devmajor and
Devminor in FileInfoHeader. Code based on the Major/Minor functions for
Darwin in golang.org/x/sys/unix.

Change-Id: I51b65f607bfa2e6b177b8b66e2b246b771367b84
Reviewed-on: https://go-review.googlesource.com/60850Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/archive/tar/stat_unix.go

@@ -61,6 +61,11 @@ func statUnix(fi os.FileInfo, h *Header) error {
 			minor := uint32((dev & 0x00000000000000ff) >> 0)
 			minor |= uint32((dev & 0x00000ffffff00000) >> 12)
 			h.Devmajor, h.Devminor = int64(major), int64(minor)
+		case "darwin":
+			// Copied from golang.org/x/sys/unix/dev_darwin.go.
+			major := uint32((dev >> 24) & 0xff)
+			minor := uint32(dev & 0xffffff)
+			h.Devmajor, h.Devminor = int64(major), int64(minor)
 		default:
 			// TODO: Implement others (see https://golang.org/issue/8106)
 		}