RFR 8157517 : jshell tool: allow undoing operations

[Robert Field]

Please review ...

Bugs:

     8157517 : jshell tool: allow undoing operations
     https://bugs.openjdk.java.net/browse/JDK-8157517

     8157395: jshell tool: allow the position of options on commands to 
be more flexible
     https://bugs.openjdk.java.net/browse/JDK-8157395

     8157393: jshell tool: change /set newmode ... to be consistent with 
/retain mode
     https://bugs.openjdk.java.net/browse/JDK-8157393

Webrev:

     http://cr.openjdk.java.net/~rfield/8157517v0.webrev/

Thanks,
Robert