os: reject Rename("old", "new") where new is a directory

Unix rejects this when new is a non-empty directory.
Other systems reject this when new is a directory, empty or not.
Make Unix reject empty directory too.

Fixes #14527.

Change-Id: Ice24b8065264c91c22cba24aa73e142386c29c87
Reviewed-on: https://go-review.googlesource.com/31358


Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>