rmiregistry should warn if it loads any classes from CLASSPATH. (Of course, it
should be possible to turn it off, but the default should be on in the registry.)

More generally, the LoaderHandler should have a general facility to enable logging
of classloading attempts, showing at least the name of the class sought, the
codebase being tried (if not found in CLASSPATH), the error detail (if not found
at all), and the success/failure of the whole attempt.

This would be useful for debugging. Selection of available log parameters and
suitable defaults is a smaller problem. Changing the default (from
RegistryImpl.main) to cause warnings about found-in-classpath events is important
to help newbies past a particularly awkward error.