emacs: 'C-u M-x guix-edit' prompts for directory.

* emacs/guix-base.el (guix-read-directory): New procedure.
(guix-find-location, guix-edit): Add optional 'directory' argument.
* emacs/guix-ui-package.el (guix-package-list-edit)
(guix-output-list-edit): Likewise.
* doc/emacs.texi (Emacs Commands): Mention "C-u".