When an interface goes down - e.g. cable unplugged - the DHCPv6 assigned IP address is not removed from said interface.
We should provide the same behavior here with IPv4 and IPv6 and the IPv6 address should be removed and dhcpv6 client restarted once the interface goes to operational up again.
After this implementation the process call chain would be:
```
root 2452 0.0 0.0 2484 1024 ? Ss 21:11 0:00 /sbin/netplugd -c /etc/netplug/netplugd.conf -p /run/netplugd.pid
root 3611 0.0 0.0 6936 3200 ? S 21:15 0:00 \_ /bin/sh /etc/netplug/netplug eth1.1111 out
root 3613 0.0 0.0 2472 1280 ? S 21:15 0:00 \_ run-parts --arg eth1.1111 --arg out /etc/netplug/linkdown.d
root 3615 0.0 0.0 6936 3328 ? S 21:15 0:00 \_ /bin/sh /etc/netplug/linkdown.d/vyos-python-helper eth1.1111 out
root 3619 0.6 1.0 55572 43448 ? S 21:15 0:00 \_ /usr/bin/python3 /etc/netplug/vyos-netplug-dhcp-client eth1.1111 out
root 3700 0.0 0.1 16448 5888 ? S 21:15 0:00 \_ systemctl stop dhcp6c@eth1.1111.service
```