RFR 8144095: JShell tool: normalize command parameter hand

[Robert Field]

Fixes for: 
8144095 <https://bugs.openjdk.java.net/browse/JDK-8144095>: JShell tool: normalize command parameter handling
8140265 <https://bugs.openjdk.java.net/browse/JDK-8140265>:  jshell tool: /save saves rejected input

Please review webrev:
   http://cr.openjdk.java.net/~rfield/8144095v1/ <http://cr.openjdk.java.net/%7Erfield/8144095v1/>


Jan, thanks for the pre-review --

J> Should the "/classes /methods /vars" be separated with a comma? 

R> OK

J> SnippetPredicate: Note that an alternative would be to mark the nonEmptyStream metod with @SafeVarArgs.
J> Having the specialized  interface seems nicer, in a sense. So, I guess I'd keep that, maybe just change "work-around" to "Avoid".

R> Done

-Robert