WIP: Support sup-config from files, from files for radius classes (vs. "login" group), and from radius attributes (#265)

In the end the lookup order for sup-config is (later configs can overwrite/append to previous ones):

• config-per-group/$group
• config-per-user/$user
• if groupconfig is active in radius auth:
  • for each radius class:
    • config-per-radius-class/$class
    • config-per-radius-class/group/class
  • other radius attributes

Also allows to separate the login group (aka tunnel-group) from the radius classes by checking the login group against the configured select-groups.