Partly fixes <http://bugs.gnu.org/20720>
Reported by Alex Kost <alezost@gmail.com>.
* gnu/system.scm (etc-directory): Change default #:sudoers value to a
'plain-file'. Don't bind it. Remove #~#$.
(maybe-string->file): New procedure.
(operating-system-etc-directory): Use it.
(%sudoers-specification): Use 'plain-file'.
* doc/guix.texi (operating-system Reference): Adjust accordingly.