all: make conn.Bind.Open return a slice of receive functions

Instead of hard-coding exactly two sources from which
to receive packets (an IPv4 source and an IPv6 source),
allow the conn.Bind to specify a set of sources.

Beneficial consequences:

* If there's no IPv6 support on a system,
  conn.Bind.Open can choose not to return a receive function for it,
  which is simpler than tracking that state in the bind.
  This simplification removes existing data races from both
  conn.StdNetBind and bindtest.ChannelBind.
* If there are more than two sources on a system,
  the conn.Bind no longer needs to add a separate muxing layer.

Signed-off-by: Josh Bleecher Snyder <josharian@gmail.com>

conn/bind_windows.go

@@ -266,7 +266,7 @@ func (bind *afWinRingBind) Open(family int32, sa windows.Sockaddr) (windows.Sock
 	return sa, nil
 }
 
-func (bind *WinRingBind) Open(port uint16) (selectedPort uint16, err error) {
+func (bind *WinRingBind) Open(port uint16) (recvFns []ReceiveFunc, selectedPort uint16, err error) {
 	bind.mu.Lock()
 	defer bind.mu.Unlock()
 	defer func() {
@@ -275,30 +275,30 @@ func (bind *WinRingBind) Open(port uint16) (selectedPort uint16, err error) {
 		}
 	}()
 	if atomic.LoadUint32(&bind.isOpen) != 0 {
-		return 0, ErrBindAlreadyOpen
+		return nil, 0, ErrBindAlreadyOpen
 	}
 	var sa windows.Sockaddr
 	sa, err = bind.v4.Open(windows.AF_INET, &windows.SockaddrInet4{Port: int(port)})
 	if err != nil {
-		return 0, err
+		return nil, 0, err
 	}
 	sa, err = bind.v6.Open(windows.AF_INET6, &windows.SockaddrInet6{Port: sa.(*windows.SockaddrInet4).Port})
 	if err != nil {
-		return 0, err
+		return nil, 0, err
 	}
 	selectedPort = uint16(sa.(*windows.SockaddrInet6).Port)
 	for i := 0; i < packetsPerRing; i++ {
 		err = bind.v4.InsertReceiveRequest()
 		if err != nil {
-			return 0, err
+			return nil, 0, err
 		}
 		err = bind.v6.InsertReceiveRequest()
 		if err != nil {
-			return 0, err
+			return nil, 0, err
 		}
 	}
 	atomic.StoreUint32(&bind.isOpen, 1)
-	return
+	return []ReceiveFunc{bind.receiveIPv4, bind.receiveIPv6}, selectedPort, err
 }
 
 func (bind *WinRingBind) Close() error {
@@ -395,13 +395,13 @@ func (bind *afWinRingBind) Receive(buf []byte, isOpen *uint32) (int, Endpoint, e
 	return n, &ep, nil
 }
 
-func (bind *WinRingBind) ReceiveIPv4(buf []byte) (int, Endpoint, error) {
+func (bind *WinRingBind) receiveIPv4(buf []byte) (int, Endpoint, error) {
 	bind.mu.RLock()
 	defer bind.mu.RUnlock()
 	return bind.v4.Receive(buf, &bind.isOpen)
 }
 
-func (bind *WinRingBind) ReceiveIPv6(buf []byte) (int, Endpoint, error) {
+func (bind *WinRingBind) receiveIPv6(buf []byte) (int, Endpoint, error) {
 	bind.mu.RLock()
 	defer bind.mu.RUnlock()
 	return bind.v6.Receive(buf, &bind.isOpen)
@@ -482,6 +482,8 @@ func (bind *WinRingBind) Send(buf []byte, endpoint Endpoint) error {
 }
 
 func (bind *StdNetBind) BindSocketToInterface4(interfaceIndex uint32, blackhole bool) error {
+	bind.mu.Lock()
+	defer bind.mu.Unlock()
 	sysconn, err := bind.ipv4.SyscallConn()
 	if err != nil {
 		return err
@@ -500,6 +502,8 @@ func (bind *StdNetBind) BindSocketToInterface4(interfaceIndex uint32, blackhole
 }
 
 func (bind *StdNetBind) BindSocketToInterface6(interfaceIndex uint32, blackhole bool) error {
+	bind.mu.Lock()
+	defer bind.mu.Unlock()
 	sysconn, err := bind.ipv6.SyscallConn()
 	if err != nil {
 		return err