That chokes on client-hook and privproc.

This is consistent with the semantics of the daemon: it resolves the
server address asynchronously. Indeed, DNS may not be available when
checking the configuration.

And revert to off by default for security and backward compatibility
reasons.

On the one hand, it might be such a great idea to join more than one.
On the other hand, we do not track all interfaces anyway.

(Maybe we should send bubbles only on the default interface too.)

(rather than those that existed when the the tunnel got up)

Fixes startup and cleanup order of processing threads.

Process the incoming router advertisement in the caller thread.