RADIUS authentication module improvements

Current RADIUS Web-interface authentication scheme lacks support of user-level authorization, all users are granted privilege 10, the maximum one.

Attached file, `radius.inc.php.patch`, containing full replacement for `radius_authenticate` and `radius_auth_user_level` functions. These functions now allow to use RADIUS-IETF attribute Class (attr. #25) to be used for assigning privilege levels to users in the web interface. My setup now uses these functions to assign either privilege 1 or 10 to users and seems to work fine. If you'll find this addition useful, please add it to Observium code. Or maybe you'll find essential to make new variable, like `$config['auth_radius_auth_level_attr']` for users to be able to choose attr. #, affordable for them and not just attr. #25.