Passing `--cipher none` to OpenVPN is not enough anymore because
clients and servers can still negotiate the algorithm to use for
encryption (by default not empty). We pass the option `--ncp-disable`
to disable cipher negotiation.

/reviewed-on !12

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.

There is no plan for a default ipv4 route.

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

We'll have to revive UDP because we experienced congestion with TCP.
This should make UDP efficient in good environment.
MTU discovery is required however to enable UDP by default.

256 * (1 - (1-42/256.)**(15/4.)) ≈ 125
where:
- 42 is default rtt-decay
- 4 is default hello in babeld
- 15 is default hello in re6st

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.

The previous broadcast model is replaced by a query-response one.
During normal operation, the cache of peers is not used anymore to select
peers to connect to. It now only used for bootstrapping and avoid querying
an already known address.

Currently, a peer broadcast its address to evrybody but this should change