emacs: Improve buffer procedures.

* emacs/guix-base.el (guix-list-or-info-buffer-p): Delete.  Replace with...
  (guix-buffer-p): ... this.  New procedure.
  (guix-buffers): Use it.  Add 'modes' argument.
  (guix-update-buffer): New procedure.
  (guix-update-buffers-maybe): Rename to ...
  (guix-update-buffers-maybe-after-operation): this.  Use
  'guix-update-buffer'.