Safecracker select list support

[GDmac]

Support for select list with required category.
Option value="0" or value="" would normally pass validation.
this pull request checks if count==1 AND count[0] == empty
Selecting the "Choose..." item won't pass validation anymore.

        <select name="category[]" id="categories">
            <option value="">Choose...</option>
            {categories}
            <option value="{category_id}"{selected}>{category_name}</option>
            {/categories}
        </select>