Currently the engine documents all exposed options through the description extension; this has some downsides:

* when the JVM wants to change engine option default, -XX:CRaCEngineOptions=help would print wrong value
* options completely controlled by JVM (such as image_location) are reported, too
* options documentation format is fully up to the engine (and it would be more complicated to reflow lines in the output)

The solution is exposing options info in a more structured manner.

As an extra feature we can use -XX:CRaCEngineOptions=help=foo to limit help to only those containing 'foo' in the name.