Also, add iptables/ip6tables example configuration.

Required to share the connectivity with others.

The list of authorized IPs for private RPCs is now configurable.
This is required when the registry is not bound to localhost.

With this API, a client can query IPv6/IPv4 information outside re6stnet/babel
for reporting.

The API considers that the email is unique, else it returns the first
occurrence. For SlapOS integration, it is more than enough to consider
that email will not be repeated.

    addToken, isToken and DeleteToken are introduced to manage tokens created
    by other system (like SlapOS for example).

If token is present, raise conflict instead loop until timeout.

This is a useful class that can be used for other reasons.

Which happens when only re6stnet is restarted. The DIF must even be recreated
if the local certificate has changed, so that we can reconfigure the shim DIF.

Commit 6c4a5443 fixes a routing issue and
as usual, all nodes without exception must either route correctly or die:
in the future, we may want to force all nodes to upgrade.

"passive mode" means that re6stnet is configured to not create OpenVPN tunnels.

With :Author:, people may think I am the only author (which is not the case,
as shown by the Git history), whereas I just wanted to specify who is the lead
developer, i.e. the person to contact. And it's not a place to list here people
that are not involved anymore in the project.

- registry: make --dh mandatory
- node: retry if the registry returns nothing (instead of writing an empty file)