RFR 8080843: JShell tool: invalid key error occurs when external editor is used

[Robert Field]

Bug: (Note: I've put in the issue comments a description of the cause of 
the problem with references)

     https://bugs.openjdk.java.net/browse/JDK-8080843

Webrev:

     http://cr.openjdk.java.net/~rfield/8080843v0.webrev/

Thanks,
Robert

P.S.  While there, I've updated the generated file names and added code 
comments