Merge changes made in Gnus trunk.

Merge changes made in Gnus trunk.

registry.el, gnus-registry.el: Use `ignore-errors' instead of third argument NOERROR for `require', since XEmacs 21.4 does not support it.
registry.el (initialize-instance): Change :after to :AFTER to be compatible with old EIEIO version in XEmacs.
gnus-registry.el (gnus-registry-post-process-groups)
 (gnus-registry--split-fancy-with-parent-internal): Fix splitting bugs and provide better messaging.
gnus-registry.el: Load ERT unconditionally anyway, discarding errors.
registry.el: Load ERT unconditionally anyway, discarding errors.