emacs: Split 'guix-root-map' keymap.

* emacs/guix-base.el: (guix-root-map): Split into...
  (guix-buffer-map): ... this and...
* emacs/guix-ui.el (guix-ui-map): ... this.
  (guix-ui-define-interface): Generate
  'guix-ENTRY-TYPE-BUFFER-TYPE-mode-map' keymaps inheriting them from
  'guix-ui-map' and 'guix-BUFFER-TYPE-mode-map'.
* emacs/guix-info.el (guix-info-mode-map): Use 'guix-buffer-map'.
* emacs/guix-list.el (guix-list-mode-map): Likewise.
* doc/emacs.texi (Emacs Keymaps): Document new keymaps.