Merge changes made in Gnus trunk.

Merge changes made in Gnus trunk.

gnus-registry.el (gnus-registry-fixup-registry): New function to fixup the parameters that can be customized by the user between save/read cycles.
 (gnus-registry-read): Use it.
 (gnus-registry-make-db): Use it.
 (gnus-registry-spool-action, gnus-registry-handle-action): Fix messaging.
 (gnus-registry--split-fancy-with-parent-internal): Fix loop.  Map references to actual group names with sender and subject tracking.
 (gnus-registry-post-process-groups): Use `cond' for better messaging.
 (gnus-registry-usage-test): Add subject lookup test.
registry.el (registry-db, initialize-instance): Set up constructor instead of :initform arguments for the sake of older Emacsen.
 (registry-lookup-breaks-before-lexbind): New method to demonstrate pre-lexbind merge bug.
 (registry-usage-test): Use it.
 (initialize-instance, registry-db): Move the non-function initforms back to the class definition.
registry.el: New library to manage gnus-registry-style data.
gnus-registry.el: Use it (major rewrite).
nnregistry.el: Use it.
spam.el: Use it.