Guest users have comment rights enabled by default

Steps to reproduce:

• grant programming rights to "XWikiAdminGroup" on the wiki
• Un-check the Comment right for XWIkiAdminGroup and XWikiAllGroup
• check the options "Prevent unregistered users from editing pages, regardless of the page or space rights" and "Require unregistered users to solve a captcha when posting a comment on a page"
• add a comment as "Guest" and see the page author changes to "Unknown user"
• log in as admin and edit the "Watchlist" and see that the author is still "XWiki Guest"