Add new package ipfwdstate that implements reference counting for IP forwarding
state management. This allows multiple usage to safely request IP forwarding
without interfering with each other.
This specifically concerns the established/related rule since this one is not compatible with iptables-nft even if it is generated the same way by iptables-translate.
- Update nftables library to v0.2.0
- Mark traffic that was originally destined for local and applies the input rules in the forward chain if said traffic was redirected (e.g. by Docker)
- Add nft rules to internal map only if flush was successful
- Improve error message if handle is 0 (= not found or hasn't been refreshed)
- Add debug logging when route rules are added
- Replace nftables userdata (rule ID) with a rule hash
