RFR 8192863: jshell tool: /<id><tab> gives "No such command"

[Robert Field]

Please review the fix to this bug.

Note: a couple of other things were fixed or given improved readability 
in this process:

     * An incorrect trim() was removed which would cause a slash with a 
leading space to be considered a command for the purpose of a completion
     * Tab completion only special-cased "/help foo" but not "/he foo" 
or "/? foo" -- this was generalized
     * Patterns rather than on-the-fly reg-ex are now used in 
commandDocumentation()
     * Subclasses of UITesting were made more readable by substituting 
named strings for Unicode and using a single call for resource retrieval.

Bug:

     https://bugs.openjdk.java.net/browse/JDK-8192863

Webrev:

     http://cr.openjdk.java.net/~rfield/8192863v0.webrev/

Thanks,
Robert