Back-port disabled checkboxes fix from 7.x

Did this backport a couple months ago, but forgot to upload the patch. I've had this patch running on my local 6.x-dev instance for that time, haven't noticed any problems so far...

Makes total sense and makes form_type_checkbox_value() of D6 on par with D7.

How is the rest of the original issue is not applying here? I am not getting what was the issue there, the description was very bad to say the least.

Drupal.org suffers from this bug, see #378898: Resubscribe users to Maintainer News. Can someone please enlighten me on my question, so we can get this in and deployed on drupal.org sooner then later?

Gabor: the only issue here is: disabled checkboxes, even where they are checked, are not sent by the browser (by spec). We need to special case the disabled case so as to provide the correct value to form handlers.

Committed to Drupal 6.