V2 Netbird integration - UDP MIMO/SIMO, ICMPv6, DHCPv6 implementations (#106)

* begin adding udp.MuxHandler

* add udp MuxHandlerSIMO/MIMO

* add tcp rx shutdown

* icmpv6 client

* icmpv6 Client shared NDP/Echo preparation

* icmpv6 client ndp/echo split

* icmpv6 client ndp/echo split done

* icmpv6 adjustments

* add dhcpv6 stubs

* dhcpv4 preliminary revision

* add dns.NextLabel

* dns label name tweaks

* dns begin work on TCP client

* add dnstcp package

* apply gofmt changes

* add udp mux tests

* clean up, remove StackBig for now

* remove dnstcp so as to merged confident parts and we continue dnstcp work elsewhere

dns/definitions.go

@@ -238,6 +238,17 @@ const (
 	RCodeRefused RCode = 5 // refused
 )
 
+// StateClientQuery is the lifecycle state of a single DNS query.
+type StateClientQuery uint8
+
+const (
+	CQueryIdle        StateClientQuery = iota // no active query (zero value)
+	CQueryPending                             // query built, not yet transmitted
+	CQueryOutstanding                         // transmitted; awaiting response (RFC 7766 §9.3)
+	CQueryDone                                // response received and decoded
+	CQueryAborted                             // query abandoned (connection error or caller abort)
+)
+
 func b2u8(b bool) uint8 {
 	if b {
 		return 1