- new -V/--version command line option
- protocol extended to get the version of any node in the network,
  which will allow to track those running an old version of re6st

It's already in 're6stnet_subnet' and it's useful to bind to 're6stnet_ip'
without having to edit it first.

Some servers can only be accessed via their re6st IP. re6st itself uses little
memory so it should not die when it fails to fork.

This is a workaround waiting that we have better criteria to select tunnels
to create or destroy.

UDP protocol is useless if nothing is done to prevent fragmentation.
Otherwise, it is at best unefficient.

There exist routers on the internet that filter fragmented packets with specific
data. This is hard to debug because TCP connections hang randomly when there is
no OpenVPN encryption.

Now, only TCP is enabled by default. A second protocol should be there for
better performance when possible, either existing UDP one (provided it is
guaranteed there is no fragmentation) or something better (GRE ?).

The way peer addresses were exchanged polluted caches with information about
dead nodes. In particular, bootstrapping often took a long time because the
cache of the primary node was mostly useless.

This also fixes bootstrap of registry.

This is a common misconfiguration that may break internet acces for other peers.

We also stop checking for child process termination when used without tunnel
manager (i.e. with --client or --client-count=0) because it conflicts with the
'ip route' command that is called every minute if --table=0 is used.
Anyway, with a tunnel manager, only openvpn client are watched.

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

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

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

Also provides systemd units.

It's normally safe to use --table=0 because most nodes use SLAAC and by default,
any existing default route is deleted.
It's better to abort if someone who explicitely wants to use a separate table
whereas the kernel is limited.