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.

Author: cmb69

main/network.c

@@ -1171,16 +1171,18 @@ PHPAPI void _php_emit_fd_setsize_warning(int max_fd)
 PHPAPI int php_poll2(php_pollfd *ufds, unsigned int nfds, int timeout)
 {
 	fd_set rset, wset, eset;
-	php_socket_t max_fd = SOCK_ERR;
+	php_socket_t max_fd = SOCK_ERR; /* effectively unused on Windows */
 	unsigned int i;
 	int n;
 	struct timeval tv;
 
+#ifndef PHP_WIN32
 	/* check the highest numbered descriptor */
 	for (i = 0; i < nfds; i++) {
 		if (ufds[i].fd > max_fd)
 			max_fd = ufds[i].fd;
 	}
+#endif
 
 	PHP_SAFE_MAX_FD(max_fd, nfds + 1);