system: 'hosts-file' is now a file-like object.

Partly fixes <http://bugs.gnu.org/20720>.
Reported by Alex Kost <alezost@gmail.com>.

* gnu/system.scm (default-/etc/hosts): Change 'text-file' to 'plain-file'.
  (maybe-file->monadic): New procedure.
  (operating-system-etc-directory): Use it.
* doc/guix.texi (operating-system Reference, Networking Services): Adjust
  accordingly.