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

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>