Add batched UDP I/O on Darwin via sendmsg_x/recvmsg_x

On a connected socket sendmsg_x sends a whole batch in one syscall;
msg_name is unsupported there, so batched sends require connecting to
the peer, which loses roaming and is therefore opt-in via
SetSinglePeerMode for single fixed-peer binds. recvmsg_x fills in
per-message source addresses, so batched receive works on unconnected
sockets too. Any unexpected errno permanently falls back to the generic
paths. iOS is excluded: in the Network Extension recvmsg_x on
unconnected UDP sockets delivers no data, and connected sockets stop
passing traffic after a rebind.

conn/msgx_default.go

@@ -0,0 +1,30 @@
+//go:build !darwin || ios
+
+package conn
+
+import (
+	"net"
+
+	"golang.org/x/net/ipv6"
+)
+
+const supportsMsgX = false
+
+const msgXBatchSize = 1
+
+type msgXState struct{}
+
+func (m *msgXState) reset() {
+}
+
+// SetSinglePeerMode is a no-op on platforms without sendmsg_x/recvmsg_x.
+func (s *StdNetBind) SetSinglePeerMode() {
+}
+
+func (s *StdNetBind) sendMsgX(conn *net.UDPConn, msgs []ipv6.Message) (bool, error) {
+	return false, nil
+}
+
+func (s *StdNetBind) makeReceiveMsgX(conn *net.UDPConn, isV6 bool) (ReceiveFunc, error) {
+	panic("makeReceiveMsgX is not supported on this platform")
+}