Back-porting some fixes from the D5 port.
Security fixes and back-porting from Drupal 5 version.
- Mainly adding check_plain() to watchdog entries to prevent script injection in logs.
- Added missing check_markup() to theme_classified_body()
- Got rid of verbose logging of date change on hook_submit()
- Stats block requires access log enabled.
- Show 'suggest new category' only if contact module is enabled.