os: consider only files from #M as regular on Plan 9

TestRemoveDevNull was added in CL 31657. However, this test
was failing on Plan 9, because /dev/null was considered as
a regular file.

On Plan 9, there is no special mode to distinguish between
device files and regular files.

However, files are served by different servers. For example,
/dev/null is served by #c (devcons), while /bin/cat is served
by #M (devmnt).

We chose to consider only the files served by #M as regular
files. All files served by different servers will be considered
as device files.

Fixes #17598.

Change-Id: Ibb1c3357d742cf2a7de15fc78c9e436dc31982bb
Reviewed-on: https://go-review.googlesource.com/32152
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/os/stat_plan9.go

@@ -31,6 +31,10 @@ func fileInfoFromStat(d *syscall.Dir) FileInfo {
 	if d.Mode&syscall.DMTMP != 0 {
 		fs.mode |= ModeTemporary
 	}
+	// Consider all files not served by #M as device files.
+	if d.Type != 'M' {
+		fs.mode |= ModeDevice
+	}
 	return fs
 }