finish PPP server mode support

[wkornewald]

Servers should authenticate themselves before the client (if requested) in order to make sure it's trustworthy.

If you want a server that handles multiple connections you should probably extend KPPPDevice such that it can wait for incoming connections and automatically create KPPPInterface instances for them.

Find a very easy solution for servers handling multiple connections. It must support RADIUS, LDAP, and other authentication schemes which might mean that it has to communicate with a userland service (but this can be part of the authenticator module which could start its own userland server via load_image()).