Create license checker for the JDK sources

The JDK sources have various implicit definitions of correct licenses which vary by repository. An approximate summary:

* C++ sources in the HotSpot repo should be under GPL
* Java sources in the langtools repo should be under GPL w/ ClassPath Exception
* Tests in the jdk and langtools repos should be under GPL, except for langtools tests with an explicit "no dynamic copyright" statement

and so on.

A shell script or Java program should be created to check that invariants. The checker can build on the license templates already checked into for forest.