RFR 8163840 (v3): jshell tool: provide way to display configuration settings

[Robert Field]

Please review this change which allows all settings to be shown.
Revamped as discussed in the thread "JShell: showing and retaining 
settings" (copied into bug Description).

Bug:
     https://bugs.openjdk.java.net/browse/JDK-8163840

Webrev:
     http://cr.openjdk.java.net/~rfield/8163840v3.webrev/

Thanks,
Robert