services: shepherd: Default to version 0.9.

* gnu/services/shepherd.scm (scm->go): Define 'shepherd&co' and pass it
to 'with-extensions'.
(shepherd-configuration-file): Call 'start-in-the-background' when it is
defined.
(<shepherd-configuration>)[shepherd]: Default to SHEPHERD-0.9.
* gnu/system.scm (hurd-default-essential-services): Use SHEPHERD-0.8.

gnu/system.scm

@@ -765,7 +765,10 @@ bookkeeping."
           %boot-service
           %hurd-startup-service
           %activation-service
-          %shepherd-root-service
+          (service shepherd-root-service-type
+                   (shepherd-configuration
+                    (shepherd shepherd-0.8)))     ;no Fibers
+
           (service user-processes-service-type)
           (account-service (append (operating-system-accounts os)
                                    (operating-system-groups os))