emacs: Remove "M" key binding for marking all lines.

* emacs/guix-list.el: Split 'guix-list-mark' into internal and
  interactive functions.  Remove "M" key binding, use "C-u m" instead.
  (guix-list--mark): New function.  Use it in other marking functions.
  (guix-list-mark): Mark all lines with a prefix argument.
* doc/emacs.texi (Emacs List buffer): Document changes in the key
  bindings.