Use "U" to upgrade the current package, "^" to upgrade all.

* emacs/guix-list.el: (guix-list-unmark): With prefix, mark all.
  (guix-package-list-mark-outputs): New procedure.
  (guix-package-list-mark-install, guix-package-list-mark-delete)
  (guix-package-list-mark-upgrade): Use it.
  (guix-package-list-mark-upgrades): New command.
* doc/emacs.texi (emacs List buffer): Update the manual accordingly.