The previous commit, which adds --ipv6, has the issue
that it does not check whether given IPs are valid.
Since IPv4 & IPv6 use completely different address
representation, --ip can be used for both.

When re6st attempts to use UPnP and IPv6 is enabled at the same time,
the external IPv4 was published for IPv6 protocols.
For example, machine6 in the demo had:
  10.0.1.3,1194,tcp;10.0.1.3,1194,udp;10.0.1.3,1195,udp6

This caused re6stnet to crash (socket.gaierror) if GEOIP2_MMDB is set.

With this commit, IPv4 & IPv6 are now processed independently.

/reviewed-on !11

This simplify network configuration a lot, and on recent kernels, this fixes
wrong source address for extra interfaces that already have a public IP.

Generating them takes a lot of time and there's no reason to do this by default.
We keep --dh option in 're6stnet' to not break existing configuration.

We consider using sockets to communicate with OpenVPN, via --management option.

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 ?).

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.

This replaces --connection-count, for more customization, but without requiring
more configuration from the user.

Co-written with Ulysse