os: change Open(`C:`) to open current directory on C:

os: change Open(`C:`) to open current directory on C:

Open(`C:`) currently opens root directory on C:. Change that to open
current directory on C:. Just like cmd.exe's "dir C:" command does.
Just like FindFirstFile("C:*") Windows API does. It is also consistent
with what filepath.Join("C:", "a") currently does.

Fixes #13763

Change-Id: I60b6e7d80215d110bbbb6265c9f32717401638c6
Reviewed-on: https://go-review.googlesource.com/18184
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>