exec: LookPath should not search %PATH% for files like c:cmd.exe

R=golang-dev
CC=golang-dev
https://golang.org/cl/4632055

src/pkg/exec/lp_windows.go

@@ -56,7 +56,7 @@ func LookPath(file string) (f string, err os.Error) {
 		}
 		exts = append(exts, e)
 	}
-	if strings.Contains(file, `\`) || strings.Contains(file, `/`) {
+	if strings.IndexAny(file, `:\/`) != -1 {
 		if f, err = findExecutable(file, exts); err == nil {
 			return
 		}