Positional arguments fail with negative numbers

Because of https://github.com/google/argh/blob/f02f6b44444b340a553ff2db298744721ebc77a7/argh/src/lib.rs#L923

positional arguments that start with a `-` get misinterpreted as optionals. This prevents e.g. taking numbers or filenames that start with a dash as required arguments, despite being permitted by Fuchsia's command-line tools rubric.