* nnmail.el (nnmail-spool-hook): new hook

* nnmail.el (nnmail-spool-hook): new hook
(nnmail-cache-insert): call nnmail-spool-hook

* gnus-registry.el: new file with examples of using the hooks

* gnus.el (gnus-registry): added registry customization group
(gnus-group-prefixed-name): improve function to return full group
name optionally
(gnus-group-guess-prefixed-name): shortcut to
gnus-group-prefixed-name, using just the group name
(gnus-group-full-name): always get a group's full name
(gnus-group-guess-full-name): shortcut, using just the group name

* gnus-sum.el (gnus-summary-article-move-hook)
(gnus-summary-article-delete-hook)
(gnus-summary-article-expire-hook): new hooks
(gnus-summary-move-article, gnus-summary-expire-articles)
(gnus-summary-delete-article): invoke the new hooks