Debian-installer stateful DHCPv6
With our current setup, the debian-installer can't do stateful DHCPv6 properly. Symptoms:
- d-i obtains an address without issue (though we still have to wait for DHCPv4 to time out)
- when netcfg exits, it kills dhcp6c, which then releases the DHCP lease and removes the address from the interface
- in theory this should not happen and dhcp6c should be demonized according to https://salsa.debian.org/installer-team/netcfg/-/blob/master/autoconfig.c#L129
So, after the network autoconfig stage is done, this leaves the VM with the interface in state UP, with routing set up, and resolv.conf pointing at the DNS server, but no external connectivity beyond the gateway, and the installation can only complete if we're using a HTTP proxy on the gateway host.