RFR 8146138 et. al.: jshell tool: add /help command plus misc fixes

[Robert Field]

Fixes:

8146138 jshell tool: add /help <command>
8147495 jshell tool: correctly handle arguments on /seteditor command
8147886 jshell tool: commands don't allow reference to start-up or 
explicit id of dropped/failed snippets
8147887 jshell tool: /list start -- fails
8147898 jshell tool: /reload quiet -- should quiet echo

Webrev:

   http://cr.openjdk.java.net/~rfield/8146138v0.patch/

Thanks,
Robert