RFR 8177076: jshell tool: allow non-zero /exit

[Robert Field]

The seed of this is the ability to exit the jshell tool with a user 
specified exit code.  For example:

     /exit result > base ? 0 : 7

However, this requires the tool launching API to support passing on the 
exit code.
In turn, this requires the tool to return exit codes, and do so 
correctly for tool errors.

This is the first case of a command taking a snippet as an argument.  
Input continuation was tied to snippet input processing.
Input, continuation, command, and snippet processing needed to be 
unraveled.

Diagnostic display, formerly only needed for snippets, needed to be 
refactored.

Tab completion needed to be handled in this new case (note integer context).

On testing side, StartOptionTest and its subclass ToolProviderTest were 
a teetering pile of spaghetti that were not amenable to
the additional demands of the new needed tests.  They have been revamped.

Bugs:

     8177076: jshell tool: allow non-zero /exit
     https://bugs.openjdk.java.net/browse/JDK-8185840

     8190383: JShell API: no way for the jshell tool to report exit 
status to provider
     https://bugs.openjdk.java.net/browse/JDK-8190383

Webrev:

     http://cr.openjdk.java.net/~rfield/8185840v0.webrev/

Thanks,
Robert