Handles unread topics and replies.

• Called in one of three ways...
• index.php?action=unread - Unread Topics (since your last visit)
• index.php?action=unread;all - All Unread Topics
• index.php?action=unreadreplies - Updated Topics (topics that you've read that have been replied to since you last viewed them)
• If "board" is specified in the querystring (board=board_id), only shows items from that board
• Uses the Recent template
• Uses either the unread or the replies sub-template depending on the action
• Skips topics with polls if polls are disabled