Clarify usage of max_fd in php_poll2() on Windows

Clarify usage of max_fd in php_poll2() on Windows

Actually, `max_fd` is not used on Windows, since `PHP_SAFE_MAX_FD` and
`select` ignore it; so it makes no sense to calculate it in the loop.
Even worse, since `php_socket_t` is unsigned on Windows, it will never
be modified during the loop, because `SOCK_ERR` is already the largest
representable value of that type.

Therefore we skip the loop on Windows, and add a clarifying comment.