* gnus-registry.el (gnus-registry-install): Allow 'ask as an option.

* gnus-registry.el (gnus-registry-install): Allow 'ask as an option.
(gnus-registry-initialize, gnus-registry-install-p): Use it.
(gnus-registry-install-shortcuts): Renamed from
gnus-registry-install-shortcuts-and-menus.  Installs the shortcuts in
the `gnus-registry-mark-map' keymap dynamically from
`gnus-registry-marks'.  The generated functions update the summary line
when a registry mark is added or deleted, and will call
`gnus-registry-install-p' (see the comments in the code).
(gnus-registry-user-format-function-M): Use concat intelligently.

* gnus-sum.el (gnus-summary-make-menu-bar): Add menu entries for all
the registry mark functions.