With the router upgrade to Debian 13 the systemd version got upgraded as
well breaking the current configuration for IP forwarding.
Add a variable for global systemd-networkd configuration and use that to
enable IPv4 and IPv6 forwarding on the router.
The systemd_networkd role could be a bit nicer, not deploying/deleting
the global configuration, if the variable is empty and
reloading/restarting systemd-networkd at appropriate times. But as is
works for now.
