[client] Improve route acl (#2705)

- 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

util/net/net.go

@@ -11,7 +11,8 @@ import (
 
 const (
 	// NetbirdFwmark is the fwmark value used by Netbird via wireguard
-	NetbirdFwmark = 0x1BD00
+	NetbirdFwmark    = 0x1BD00
+	PreroutingFwmark = 0x1BD01
 
 	envDisableCustomRouting = "NB_DISABLE_CUSTOM_ROUTING"
 )