$ip: User's IP address
&$blocked: Whether the user is blocked, to be modified by the hook
+'UserIsEveryoneAllowed': Check if all users are allowed some user right; return
+false if a UserGetRights hook might remove the named right.
+$right: The user right being checked
+
'UserLoadAfterLoadFromSession': Called to authenticate users on external or
environmental means; occurs after session is loaded.
$user: user object being loaded