system: Populate /etc/shells with the list of all the shells in use.

Reported by Andy Wingo <wingo@pobox.com>.

* gnu/system.scm (user-shells, shells-file): New procedures.
  (etc-directory): Add #:shell parameter.  Use 'shells-file' instead of
  'text-file'.
  (operating-system-etc-directory): Call 'user-shells' and pass #:shells
  to 'etc-directory'.