emacs: list: Generalize 'marks' code.

* emacs/guix-list.el (guix-list-mark-alist): Rename to...
  (guix-list-marks): ... this
  (guix-list-data, guix-list-default-marks): New variables.
  (guix-list-value, guix-list-additional-marks, guix-list-marks): New
  procedures.
  (guix-list-define-entry-type): Adjust accordingly.