"You are not authorised..." still sends HTTP status 200 instead of i.e. 403

When not being able to read a forum, a topic or e-mailing a topic phpBB currently displays an error message like "You are not authorised to xxx". However, the HTTP status code sent is still 200, which is not that trivial for (search engine) bots to understand.

A HTTP status code 403 should be used instead, as it is already done in /download/file.php:

send_status_line(403, 'Forbidden');

This line should be added to the following files aswell:

• /memberlist.php (2 times)
• /viewforum.php (1 time)
• /viewtopic.php (1 time)