403 flag for _check_auth + fix default 404 page
403 flag for _check_auth + fix default 404 page

_check_auth can now return a 403 error instead of redirected to
the login page if needed (in case of the checkppolicy for example).
The default page handles post data and querystring better now.

kakwa/ldapcherry,kakwa/ldapcherry,kakwa/ldapcherry