fix(firewall): flush conntrack table after enabling firewall at container start

- prevent leaks for connections made the first ~10 milliseconds when Gluetun starts
- seems critical,  but in practice this very rarely happen and it very hard to reproduce

internal/netlink/conntrack_unspecified.go

@@ -0,0 +1,7 @@
+//go:build !linux
+
+package netlink
+
+func (n *NetLink) FlushConntrack() error {
+	panic("not implemented")
+}