os: fix IsPermission on windows

Fixes #3482.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5987070

src/pkg/os/error_windows.go

@@ -26,5 +26,5 @@ func isPermission(err error) bool {
 	if pe, ok := err.(*PathError); ok {
 		err = pe.Err
 	}
-	return err == ErrPermission
+	return err == syscall.ERROR_ACCESS_DENIED || err == ErrPermission
 }