Publication Date module integration

Since Drupal 6 was EOL-ed in 02/2016, no additional fixes will be added: this issue was already 8 months late when it was opened, so marking won't fix.

On a side note, three not-optimal practices here:

• you typically don't want to use module_invoke() manually to call a specific function: just calling it directly is faster. Hooks are for calls not known in advance
• more generally, hook_nodeapi should be triggered by core during node saves, but not used manually.
• the call pattern is inverted : It is not for an event source (here Classified Ads) to invoke its users, but for users to register for an event (hook in Drupal <= 7, event_listener on Drupal >= 8) and the source to just emit the event and let its listeners do whatever they want : this reduces coupling, hence brittleness.