Send HTTP 404 if topic, forum or user do not exist

Currently phpBB shows a "Topic doesn't exist" message when the user tries to open an invalid URL, the URL of a deleted topic or the URL of a topic that's inaccessible to this user. The same goes for forums.

For bots it would be good if a 404 is returned in these cases because:

• This makes it easier to erase unwanted information from search engine caches/indices
• Some bots don't like the fact that random URLs return a 200 statuscode
• The bot won't need to update the URL (as often) as it would when a 200 is returned
• It's not very useful to index a "Topic doesn't exist" page

I've fixed this by adding this around the topic of msg_handler():

        if ($msg_text == 'NO_TOPIC' || $msg_text == 'NO_FORUM')

        {

                header("HTTP/1.0 404 Not Found");

        }

IMO this is not the cleanest way to fixed this but this saved me quite a bit of work Those two messages are returned in quite a few places...