More flexibility for form_checkboxes and form_radios attributes

The attributes argument of form_checkboxes() and form_radios() is currently used to apply the same attributes to each input tag. In some cases you might want to have different attributes applied to each checkbox or radio button. For example, this makes disabling some of the checkboxes possible.

An example of the usage is

array('somekey' => array('disabled' => 'disabled'), 'someotherkey' => (...), ...)

This patch also removes the br tag after each checkbox and radio button. Drupal.css is used to set the display to block. br is a presentational tag which often conflicts with using CSS to style things.