Rework setting/unsetting of gnus-registry-enabled

Rework setting/unsetting of gnus-registry-enabled

The basic idea is, we're moving the setting/unsetting of
`gnus-registry-enabled' so that it is a more reliable indicator of
whether the registry is actually available or not.

* lisp/gnus/gnus-registry.el (gnus-registry-load): Move the setting of
`gnus-registry-enabled' here, after we're sure the registry actually
has been loaded.
(gnus-registry-initialize): Move it out of here. All this function
does is conditionally call `gnus-registry-load'.
(gnus-registry-register-message-ids): Only check
`gnus-registry-enabled', since we'd more confident about this
now. Sort checks in order of increasing complexity.
(gnus-registry-clear): Unset `gnus-registry-enabled'.
(gnus-registry-install-hooks): This function should only install
hooks, not set variables.
(gnus-registry-unload-hook): Change the unload hook to call
`gnus-registry-clear', as that will do all the necessary
work (including calling `gnus-registry-unload-hook')