Experiments with native OVH IPv6 aren't good

Identified as an issue with NDP, fixed in https://reviews.freebsd.org/D23695.

So it's expected that works fine on web-001 (FreeBSD 13.2, with that fix) but can't be repro on router-001 (FreeBSD 13.1, without that fix).

For T1854, I'm currently upgrading router-001 to FreeBSD 13.2.

Note that only seems to solve OUTGOING connectivity, and not ingoing connectivity. So for web-001, a tunnel is probably best.

With proper route configuration, I've in/out IPv6 connectivity for web-001 with OVH native IP.

Routing:

1. A static route to the gateway through the interface: route -6 add 2001:41d0:0303:d9ff:00ff:00ff:00ff:00ff -iface vmx1
2. A default route to the gateway: route -6 change default 2001:41d0:0303:d9ff:00ff:00ff:00ff:00ff

In term of configuration file that means, we need to provide this with Salt:

ipv6_static_routes="gw default"
ipv6_route_gw="2001:41d0:0303:d9ff:00ff:00ff:00ff:00ff -iface vmx1"
ipv6_route_default="default 2001:41d0:0303:d9ff:00ff:00ff:00ff:00ff"

We need to figure how to express the idea "GW isn't in our prefix" to use that syntax
instead of the more straightforward "ipv6_defaultrouter=..." which is the current Salt config.

FYI, ipv6_defaultrouter defines static_routes as "_default" and creates a _default route:

*)
    ipv6_static_routes="${ipv6_static_routes} _default"
    ipv6_route__default="default ${ipv6_defaultrouter}"
    ;;

(Note how this syntax solves one of the T1757 extra route issue: by including the variable even empty, we're sure not to overwrite the current routing)

Incoming connectivity:

$ ping -6 www-alkane.nasqueron.org
PING6(56=40+8+8 bytes) 2001:bc8:6005:5:aa1e:84ff:fef3:5d9c --> 2001:41d0:303:d971::517e:c0de
16 bytes from 2001:41d0:303:d971::517e:c0de, icmp_seq=0 hlim=53 time=5.707 ms
16 bytes from 2001:41d0:303:d971::517e:c0de, icmp_seq=1 hlim=53 time=5.777 ms
16 bytes from 2001:41d0:303:d971::517e:c0de, icmp_seq=2 hlim=53 time=5.682 ms
16 bytes from 2001:41d0:303:d971::517e:c0de, icmp_seq=3 hlim=53 time=5.743 ms
16 bytes from 2001:41d0:303:d971::517e:c0de, icmp_seq=4 hlim=53 time=5.758 ms

We need to figure how to express the idea "GW isn't in our prefix" to use that syntax
instead of the more straightforward "ipv6_defaultrouter=..." which is the current Salt config.

D3077 introduced a new network_utils.can_directly_be_discovered method for that.

When true, we can use ipv6_defaultrouter. When false, we use the two routes.

[ Priority high, as it's a blocker to switch ready sites - like MediaWiki and the static ones - to Alkane ]

The following routing table works like a charm on web-001:

For reference, history to experiment the configuration in D3078 is published at P328 (as we can't repro it fully to router-001).