sentinel-firewall: rules in some tables are left in place with firewall restart/reload

Tables such as mangle are not flushed once firewall is reloaded/restarted. That is problematic as we are not removing our old rules from them. It is prepared so we would do so. We can match all our rules by comment !sentinel: but that is not implemented at the moment. We should implement it in some capacity.

The only issue we have to think about is that sentinel-firewall scripts potentially create not only rules but also tables. Those tables are not marked in any way and removing them is not probably because of that possible. Having some tables in place is not in general a problem. The problem is that those tables have some rules (such as drop all traffic at the end) that should not be removed but they are marked with that !sentinel as well.