PPP auth required flag is currently hardcoded to true if PPP is
acting as a server and set to false if PPP is acting as a client.
This is probably the most wanted behavior, but since we now have the
ability to change that at runtime, allow users to do it.
It means we can now have a server which asks the client to authenticate
or vice versa. This is pretty unusual thought. What we don't support
yet is mutual authentication with a different set of user and password
per direction which is even less usual.