Jigsaw modules build

[Mark Reinhold]

Thanks for the merge last Friday.  I've pulled it into my working forest
and am rebasing all my patches today.

I'm pretty sure I already mentioned this verbally, but just in case not:
Please make it your priority to update the modules build to use Dalibor's
latest jpkg changes to generate module files for the JDK modules.  Our
goal is to create a minimal JRE base image into which additional modules
can be installed.  The legacy-mode effort can wait for now.

There are some performance issues in the module-file code, but Dalibor is
already working on them.  If you run into any other problems, please let
him know.

Thanks,
- Mark