HMAC is added in babel call to prevent babel communication between nodes of different re6st networks. 
This solves the problem of machines in different re6st networks but on the same LAN that exchange routes through babel. 
The key used to authenticate packets is randomly created on 16 bytes by the registry and sent to nodes when they fetch network parameters. 
This uses the WIP hmac branch of jech/babeld with Nexedi patches and the added possibility to not check HMAC in incoming packets for better HMAC integration on a HMAC-less network.

/reviewed-on !18

In commit 06974788,
we increased the --link-mtu value as a temporary way to compensate the
unexplained behaviour change of recent OpenVPN.

This was partly due to encryption, which was enabled despite
`--cipher none`. And it happens that the behaviour of --link-mtu only
changed for the server, with a mysterious difference of 93 bytes.

Hence the workaround to get identical tunnel MTU on both sides.

/reviewed-on !13

The fix to mark an interface as "up" and indicate its MTU was
useful for machines with a single client, because OpenVPN would fail
to configure them this way in OpenVPN 2.3. It has been fixed in 2.4
so the fix has been removed.

/reviewed-on !14

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.