Support for multi-selects and multi-checkboxes in the multipage forms template

The multipage forms template in CVS currently provides little or no support for form elements whose values must be represented as an array, i.e. multi-selects and multi-checkboxes. It is difficult, but not impossible, to add support for these elements. The category_bulkedit module (part of the category package) has successfully adopted this template with support for these elements. This feature needs to be extracted from category_bulkedit, and committed back to the original template in CVS, so that other modules can also use any and all form elements in their multipage forms.