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!