Replace use of fsockopen with the drupal API

Its about this code in recaptcha.module:

// Check if reCAPTCHA is available and show Math if not.
$connect = @fsockopen(RECAPTCHA_VERIFY_SERVER, 80);
if (!$connect) {
    return captcha_captcha('generate', 'Math', $args);
}
fclose($connect);

As fsockopen is sometimes blocked for security reasons this will always cause a fallback to math captchas on some hosting providers, even if the Drupal API is patched to use curl or whatever.
It would be best to keep it encapsulated there, give an option or don't use a fallback mechanism.