Many changes to the nftables backend which will be used in the follow-up
PR with #4421.
1. Add support for chain policy: drop/accept.
2. Properly handle match on all IPs in the set (`0.0.0.0/0` like).
3. Implement conntrack state matching.
4. Implement multiple ifname matching in a single rule.
5. Implement anonymous counters.
Signed-off-by: Andrey Smirnov <andrey.smirnov@siderolabs.com>
