To support proxy LCP negotiation.
Note: we *have* to take the auth id from the proxy answer, otherwise we would
replay previous ids, for which the client might cache the answer and thus
ignore our new challenge and just repeat their outdated answer.
Previously, when we got a call to be forwarded to another LNS and we didn't
have a tunnel to it yet, we would wait for a PAP/CHAP re-send to trigger the
creation of a session in the fresh tunnel. We do not have to wait, we can
trigger that on SCCCN ack.
When acting as LNS, we have to answer challenges with our own secret, but
when acting as LAC, we have to answer challenges with the LNS secret, not
ours.
