include: drop registry.h from public API.
Since nobody (externally) seems to use registry.h at all, it can be moved into dix-internal.
This queue is doing it step by step, for easier review, but also in case that something still needed unexpectedly. In this case only part of it could be applied, leaving the rest for further rework.
Scanned trough all driver trees, as well as tigervnc source, in order to confirm it's not used by any of them.