RFR 8223065: Add jcmd to get the listen address of the debugger

[Schmelter, Ralf]

Please review the patch which adds a jcmd to get the actual address the debugging backend is listening on.

The this value was stored in the agent property sun.jdwp.listenerAddress and currently only used by the ProcessAttachingConnector.

Additionally, the listen address is now displayed by the VM.start_java_debugging command, if a new session was started.

webrev: http://cr.openjdk.java.net/~rschmelter/webrevs/8223065/webrev.0/
bugreport: https://bugs.openjdk.java.net/browse/JDK-8223065

Best regards,
Ralf