add support for writing the ECN control message (Linux, macOS)

sys_conn_helper_darwin.go

@@ -15,6 +15,8 @@ const (
 	ipv4PKTINFO  = unix.IP_RECVPKTINFO
 )
 
+const ecnIPv4DataLen = 4
+
 // ReadBatch only returns a single packet on OSX,
 // see https://godoc.org/golang.org/x/net/ipv4#PacketConn.ReadBatch.
 const batchSize = 1