Checkbox not compatible with clientside validation

Because the "I agree to the terms" checkbox is implemented using the "checkboxes" (plural) rather than "checkbox" (singular) form api type, it is not compatible with Clientside Validation. This seemed a little silly, since there is only one checkbox, so I made a quick patch to make the implementation compatible with clientside validation. This patch also pulls the "uc_termsofservice_checkout_required" variable so that the required attribute can be enabled/disabled from the settings page.