Allow site builder to decide between radios and a multi-select widget for list fields

Right now, list.module fields will always show up in the combinations fieldset as radio buttons---but there is no reason to not have them be multi-select fields.

Goal: Have the widget depend on what widget the administrator selects for the field: For select fields, turn it into a multi-select field. For checkbox/radio buttons, turn it into checkboxes.

This is how the bpc_taxonomy module operates already.