emacs: Add 'guix-root-map'.

* emacs/guix-base.el (guix-define-buffer-type): Move common key bindings
  from here...
  (guix-root-map): ... to here.  New variable.
* emacs/guix-info.el (guix-info-mode-map): Use it.
* emacs/guix-list.el (guix-list-mode-map): Use it.
* doc/emacs.texi (Emacs Keymaps): Document it.