exec: make LookPath work even when PATHEXT env variable is not set on Windows
exec: make LookPath work even when PATHEXT env variable is not set on Windows

R=golang-dev, mattn.jp
CC=golang-dev
http://codereview.appspot.com/4559062

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

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