RMI server-side multiplex protocol should be disabled

The RMI code supports a disused protocol called the "multiplex" protocol. This was removed by JDK-4183204 in JDK 1.2.2.

The server side of the RMI protocol should be disabled and eventually removed.

A system property "sun.rmi.transport.tcp.enableMultiplexProtocol" can be added with a default of "false".

In TCPTransport.ConnectionHandler.run(), the boolean should be checked when a request is received to create a multiplexProtocol connection.
If the flag is false, it should send log the rejection and send a Nak.

For compatibility, setting the system property to true will accept and process the request as before.