Admins should see guestbooks of blocked users

Currently, the code spits out a 404 when trying to access a blocked user's guestbook. This behavior differs from Drupal core, as users with the 'administer users' permission are still allowed to view a blocked users profile. Guestbook should follow this behavior, since 'administer users' is a pretty strong permission.

It is also required for adding Panels support, which is quite easy when we're reusing guestbook_page(). In that case, however, guestbook_page() shouldn't call dnf(), because context handling will be managed by Panels.