RFR 8167637: jshell tool: /edit should use EDITOR setting

[Robert Field]

Please review --

Bugs:

     8167637: jshell tool: /edit should use EDITOR setting
     https://bugs.openjdk.java.net/browse/JDK-8167637

     8167640: jshell tool: external editor temp file should be *.java
     https://bugs.openjdk.java.net/browse/JDK-8167640

Webrev:

     http://cr.openjdk.java.net/~rfield/8167637v0.webrev/

Thanks,
Robert