services: Missing services are automatically instantiated.

This simplifies OS configuration: users no longer need to be aware of
what a given service depends on.

See the discussion at
<https://lists.gnu.org/archive/html/guix-devel/2018-01/msg00114.html>.

* gnu/services.scm (missing-target-error): New procedure.
(service-back-edges): Use it.
(instantiate-missing-services): New procedure.
* gnu/system.scm (operating-system-services): Call
'instantiate-missing-services'.
* tests/services.scm ("instantiate-missing-services")
("instantiate-missing-services, no default value"): New tests.
* gnu/services/version-control.scm (cgit-service-type)[extensions]: Add
FCGIWRAP-SERVICE-TYPE.
* gnu/tests/version-control.scm (%cgit-os): Remove NGINX-SERVICE-TYPE
and FCGIWRAP-SERVICE-TYPE instances.
* doc/guix.texi (Log Rotation): Remove 'mcron-service-type' in example.
(Miscellaneous Services): Remove 'nginx-service-type' and
'fcgiwrap-service-type' in Cgit example.

gnu/services/version-control.scm

@@ -263,7 +263,11 @@ access to exported repositories under @file{/srv/git}."
     (list (service-extension activation-service-type
                              cgit-activation)
           (service-extension nginx-service-type
-                             cgit-configuration-nginx-config)))
+                             cgit-configuration-nginx-config)
+
+          ;; Make sure fcgiwrap is instantiated.
+          (service-extension fcgiwrap-service-type
+                             (const #t))))
    (default-value (cgit-configuration))
    (description
     "Run the Cgit web interface, which allows users to browse Git