Feature/Bugfix: IPv6 blocking (#428)

- Feature/Bugfix: Block all IPv6 traffic with `ip6tables` by default
- Feature: Adapt existing firewall code to handle IPv4 and IPv6, depending on user inputs and environment
- Maintenance: improve error wrapping in the firewall package

internal/firewall/firewall.go

@@ -34,6 +34,7 @@ type configurator struct { //nolint:maligned
 	routing          routing.Routing
 	openFile         os.OpenFileFunc // for custom iptables rules
 	iptablesMutex    sync.Mutex
+	ip6tablesMutex   sync.Mutex
 	debug            bool
 	defaultInterface string
 	defaultGateway   net.IP