Continued fixes to gnus registry loading

Continued fixes to gnus registry loading

* lisp/gnus/gnus-registry.el (gnus-registry-db): Do not initialize
this variable to an empty database, that should only be done for new
databases.
(gnus-registry-load): Remove "force" argument, don't check if the
database is already loaded, as we're only going to load it once.
(gnus-registry-initialize): Either load the db directly, or set up a
hook to do it later.
(gnus-registry-install-hooks): Don't load on
gnus-read-newsrc-el-hook.