New option gnus-registry-register-all

New option gnus-registry-register-all

* lisp/gnus/gnus-registry.el (gnus-registry-register-all): If nil,
the registry won't automatically create new entries for all seen
messages. Defaults to t to preserve previous behavior.
(gnus-registry-handle-action): Don't automatically create entries; if
one doesn't exist, don't handle anything.
(gnus-registry-register-message-ids): Only register if this option is
t.
(gnus-registry-get-or-make-entry): Add optional no-create argument.
(gnus-registry-get-id-key): This "get" operation should only create an
entry if this option is t.
* doc/misc/gnus.texi: Documentation and news.