allow buttons like 'reset' to bypass validation

In Drupal 6 (for example) if you have a required text field on a form, there is now way for a button (for example 'Reset to defaults') to ignore that field even if the action to be taken does not sepend on any form inputs.

The proposed feature would allow a button to specify that the button submit handler should be executed with no validation. To minimize the security abuses this might invite, I propose that POST be emptied when this bypass is requested.