This is a follow-up to JDK-8168772. In that bug, the goal was to just refactor the generation, and keep output identical, by whatever means neccessary.

This bug takes the final step, and cleans up the code. The changes will have the following kind of effects:

1) The .options files will change, but it will not affect the generated Javadoc.

2) The generated Javadoc will change, but it is a bug fix since the old Javadoc was broken.

3) The generated Javadoc will change, but the change is minor and is just intended root out odd special cases to make all Javadoc look more the same.

On top of that are some more changes to clean up the build logic.

Here is a list of actual changes:

* DependOnVariable now handles # properly (patch by Erik!)
* Automatically determine core packages
* Bug fix: use iso-8859-1 encoding everywhere to avoid broken characters
* Bug fix: use proper link reference to coredocs from jshell
* "Invert" doclint behaviour, so we need to explicitely disable those that causes warnings
* Enable all passing doclint checks
* Fix workarounds introduced in previous Javadoc cleanup used needed to keep consistency with .options/.packages files
* Increase consistency by removing odd behavior for individual javadoc packages
* Remove [NON_]CORE_PKGS.gmk.
* Simplify hard-coded texts
* Add -notimestamp to make javadocs comparable between builds
* Ensure split index is used in all cases where the index file is >= ~200 kB.
* Clarify non-GA build titles