Java Doclet 9+ Support

[gboersma]

In Java 9, the doclet tool was completely reworked and is not backwards compatible with version 8-.

[sjoerdtalsma]

One tip to save you a lot of time (which it cost me):
If you're thinking of using the nice 'multi-release-jar' feature of Java9 for your doclet: it doesn't work.

I intended to provide two Doclet classes in the same jar, but although the javac tool understands this, the javadoc tool does not seem to get it (and tries to load the 'regular' version no matter what)..
So multiple doclet jar versions are still required (again, I learned this the hard way)..