Showing CAPTCHA description: control by an explicit checkbox instead of by unobvious string emptiness

Showing the CAPTCHA fieldset and CAPTCHA description are now controlled by the emptiness of the CAPTCHA description variable. This is unobvious and makes post module installation+configuration translation imports act weird (http://drupal.org/node/456388#comment-1640532).

It would be better to enable the fieldset and description with an explicit checkbox.
This will make usability better and it will make it possible to fix translation import weirdness.