* Returns the {@code NtlmPasswordAuthentication} object used as
     * credentials with this file or pipe. This can be used to retrieve the
     * username for example:
     * {@code
     * String username = f.getPrincipal().getName();
     * }
     * The {@code Principal} object returned will never be {@code null}
     * however the username can be {@code null} indication anonymous
     * credentials were used (e.g. some IPC$ services).