Another stab at fixing Issue #201

When no categories are delivered to serendipity_updertEntry this can mean two things:
- categories need to be un-assigned
- categories where not addressed/fetched/passed to the function [see include/admin/overview.inc.php for publishing a post! Or plugins like freetag, using serendipity_updertEntry]

To address this a new serendipity[had_categories] is added to the backend template, which can be checked for.

Feedback appreciated!

templates/2k11/admin/entries.tpl

@@ -127,6 +127,7 @@
 
                 {foreach from=$entry_vars.category_options item="entry_cat"}
                     <div class="form_check">
+                        <input type="hidden" name="serendipity[had_categories]" value="1" />
                         {$entry_cat.depth_pad}
                         <input id="serendipity_category_{$entry_cat.categoryid}" name="serendipity[categories][]" type="checkbox" value="{$entry_cat.categoryid}"{if $entry_cat.is_selected} checked="checked"{/if}>