services: Provide a 'loopback' service by default.

* gnu/services/networking.scm (static-networking-service): Add
  #:provision parameter; use it.
* gnu/services/base.scm (%base-services): Call
  'static-networking-service' for "lo".

gnu/services/base.scm

@@ -20,6 +20,7 @@
   #:use-module ((guix store)
                 #:select (%store-prefix))
   #:use-module (gnu services)
+  #:use-module (gnu services networking)
   #:use-module (gnu system shadow)                ; 'user-account', etc.
   #:use-module (gnu system linux)                 ; 'pam-service', etc.
   #:use-module (gnu packages admin)
@@ -461,6 +462,8 @@ This is the GNU operating system, welcome!\n\n")))
           (mingetty-service "tty4" #:motd motd)
           (mingetty-service "tty5" #:motd motd)
           (mingetty-service "tty6" #:motd motd)
+          (static-networking-service "lo" "127.0.0.1"
+                                     #:provision '(loopback))
           (syslog-service)
           (guix-service)
           (nscd-service)

gnu/services/networking.scm

@@ -33,6 +33,7 @@
 (define* (static-networking-service interface ip
                                     #:key
                                     gateway
+                                    (provision '(networking))
                                     (name-servers '())
                                     (inetutils inetutils)
                                     (net-tools net-tools))
@@ -49,7 +50,7 @@ gateway."
       (documentation
        (string-append "Set up networking on the '" interface
                       "' interface using a static IP address."))
-      (provision '(networking))
+      (provision provision)
       (start #~(lambda _
                  ;; Return #t if successfully started.
                  (and (zero? (system* (string-append #$inetutils