emacs: list: Generalize 'describe' code.

* emacs/guix-list.el (guix-list-describe-warning-count): Remove variable.
  (guix-list-define-entry-type): Generate
  'guix-ENTRY-TYPE-list-describe-warning-count' and
  'guix-ENTRY-TYPE-list-describe-function' variables.
  (guix-list-describe-maybe): Remove.  Move the code...
  (guix-list-describe): ... here.
  (guix-list-describe-warning-count, guix-list-describe-entries)
  (guix-list-describe-ids): New procedures.
  (guix-output-list-describe): Adjust accordingly.
  (guix-list-mode-map): Bind "i" key to 'guix-list-describe'.
  (guix-generation-list-mode-map): Remove "i" key.