See merge request !37

Commit 40d4e496 is not reverted
because Python 3 will also require to keep the distinction between
blob and text.

The behaviour of --link-mtu has changed and we increase the values to
at least have interface MTU greater than IPv6 minimum.
We'll see later to have even greater values in ovpn_link_mtu_dict
(so that the resulting MTU is closer to what we had with 2.3)
or review the whole MTU part completely.

See "./demo --help" for more information.

/reviewed-on !8

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.

Check for dead tunnels evry 5 seconds instead of waiting for the next tunnel_refresh