os: make Stdin.Stat() return ModeCharDevice if Stdin is console

os: make Stdin.Stat() return ModeCharDevice if Stdin is console

CL 20845 changed Stdin.Stat() so it returns ModeNamedPipe.
But introduced TestStatStdin does not test what Stdin.Stat()
returns when Stdin is console.

This CL adjusts both TestStatStdin and Stdin.Stat
implementations to handle console. Return ModeCharDevice
from Stdin.Stat() when Stdin is console on windows,
just like it does on unix.

Fixes #14853.

Change-Id: I54d73caee2aea45a99618d11600d8e82fe20d0c0
Reviewed-on: https://go-review.googlesource.com/34090
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>