app_isdir() cleanup

I think it's better to use `GetFileAttributes` to obtain the attributes
of a file than `FindFirstFile`. If the input name contains `*`, this
function should return failure rather than check whether the first match
happens to be a file or a directory.

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/3991)