registry.el (registry-full): Add convenience method. Fix logic.

registry.el (registry-full): Add convenience method.  Fix logic.
 (registry-insert): Use it.  Fix logic here too.
gnus-registry.el (gnus-registry-insert): Add wrapper that calls `registry-prune' if `registry-full' returns t.
 (gnus-registry-handle-action, gnus-registry-get-or-make-entry, gnus-registry-set-id-key, gnus-registry-usage-test): Use it.