With IP_MULTICAST_LOOP they can see each other. We "just" have to make sure
they use different IP addresses and route metrics to distinguish from each
other.

If it was lost, some clients (e.g. pppd) may not try to re-send their CHAP reply.

So another l2tpns on the same host can receive l2tp while we are quickly
removing our routes.

The next master can restart the authentication

So we seen when adding/removing a lot of them if that takes a long time

So we can catch up according to the window that we announce.

To be able to keep different routes from several running l2tpns instances.

To be able to distinguish routes from several running l2tpns instances.

rather than ping-ponging with radius[r].id

This also make sendchap use a different id on each call, necessary for
some clients which cache responses.

To support proxy auth.

The default 4 is really small, and imposes very sequential processing of
reconnections in case we lose all sessions. Nowadays we easily have
memory for a lot more.

controlc doesn't mean we will send more messages, but only that some of them
are pending ack. If we have already sent them and the peer has just not
acked them yet, controlnull() would not send a ZLB, and once the peer
has acked our control messages, we wouldn't send a ZLB with an updated Nr.

To support proxy auth.

If we later turn master, we want scccn already reset like when clearing a
tunnel.

It expects a boolean, not an address

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.

Which will be useful to fasten session creation after tunnel is created.

We are not really supposed to send ICRQ until we got our SCCCN acked, so better
wait for it.

To support proxy LCP negotiation.

Clients may have enabled mppp without actually using several links. We
can support acceleration in that case.

To address RadiusBLAST vulnerability.

Fixes #16