- authenticated communications with registered clients
- XML-RPC is dropped
- multi-threaded server

When a peer advertised several addresses, a node trying to create a tunnel to
it never tried any other address than the first one.

Before, we wrongly assumed OpenVPN would try all addresses before aborting
(--ping-exit). New code reexecutes OpenVPN until all addresses are tried
and update the peer db to reorder addresses if the first one failed.

'--ip any' was also broken due to missing parenthesis.

When 2 nodes were started for the first time whereas:
- one of them is in client-only mode, connected to the other one
- the registry node is temporarily down
then the normal node never tried to rebootstrap or connect directly to the
only node it knows (the registry node).
Such case required to restart the daemon when the registry is back.

Moreover, there was no reason to query the registry node immediately after
having open new tunnels to peers found in cache, when this number is less than
expected.

This reduces the probability to lose all connections at the same time when a
gateway becomes out-of-order.

This bug could even freeze re6st completely when trying to rejoin the network.

Non-numerical address should be rare and this could even be counter-productive:
such remotes are more likely to host other services and forcing routes may
break already opened non-re6st connections.

Also provides systemd units.