Add jcmd to get the listen address of the debugger

When the jdwp debugging agent starts to listen, it stores the actual address (plus the transport) in the sun.jdwp.listenerAddress agent property.

It would be beneficial for the user to be able to get this address via a jcmd, since one cannot always deduce the actual address from the address supplied by the jdwp command line.

For example, the dt_socket transport can handle port 0. It will then just listen on a free port and will store that port in the listenerAddress property.

In addition, the VM.start_java_debugging jcmd should report the actual address the VM is listening on, not just the requested one, when it started a new debugging session.