Add tcp.Listener (#15)

* begin reviewing TCP listener

* begin adding listener tests

* adding TestExchange implementation

* split out legacy and Step tests

* move listener to tcp package

* fix up tcp tests taking very long

* add xnet.TCPPool and work on TCPPool semantics

* xnet: TCPPool only accepts established connections

* xnet: getting to bottom of panic in xnet.Listener

* xnet: improve listener tests

* fix several data races in testing fixtures

* fix more synchronization things in listener test

* finalize tcplistener test

x/xnet/stack-async.go

@@ -219,6 +219,12 @@ func (s *StackAsync) Addr() netip.Addr {
 	return s.ip.Addr()
 }
 
+func (s *StackAsync) SetSubnet(subnetMask netip.Prefix) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.subnet = subnetMask
+}
+
 func (s *StackAsync) SetHardwareAddress(hw [6]byte) error {
 	s.mu.Lock()
 	defer s.mu.Unlock()
@@ -286,6 +292,16 @@ func (s *StackAsync) ListenTCP(conn *tcp.Conn, localPort uint16) (err error) {
 	return nil
 }
 
+func (s *StackAsync) RegisterListener(listener *tcp.Listener) (err error) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	lport := listener.LocalPort()
+	if lport == 0 {
+		return lneto.ErrZeroSource
+	}
+	return s.tcps.Register(listener, nil)
+}
+
 var errNoDNSServer = errors.New("no DNS server- did DHCP complete? You can set a predetermined DNS server in Stack configuration")
 
 func (s *StackAsync) StartLookupIP(host string) error {