Introduce a new hook_captcha to allow modules to define which forms have captcha points.

The attached simple patch introduces a new hook_captcha that allows modules to inform the captcha.module what captcha points need captcha.

To use this, it is very simple, for example, feedback module would have this:

function feedback_captcha() {
  return array(
    'feedback_mail_form' => t('Feedback Form'),
  );
}

Then the new captcha point will appear in captcha's settings, and the site admin can enable it.

No more editing of captcha module to add your points with each release.

This has been tested and works well.