When masking FileInfo.Mode() from a character device with the ModeType
mask, ModeCharDevice cannot be recovered.

ModeCharDevice was added https://golang.org/cl/5531052, but nothing
indicates why it was omitted from ModeType. Add it now.

Fixes #27640

Change-Id: I52f56108b88b1b0a5bc6085c66c3c67e10600619
Reviewed-on: https://go-review.googlesource.com/135075
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>