Add doclet to help when using offline links

[philwebb]

We want to move projects away from using -link in preference of -linkoffline and -javadoc.jar contents. Unfortunately javadoc doesn't emit any warnings if links are not found. We can solve that by creating our own doclet that can check links and make things easier to configure.

[philwebb]

Reopening following feedback that the link checking alone is not enough as we end up with verbose configuration.

I'm going to refine the code to allow sources to be specified and -linkoffline locations to contain comma separated lists that can be expanded.