installer: Force user to choose exactly one network management method.

Previously, for non-desktop configs, users could choose any number of
network management methods, including 0, 2, or more.

* gnu/installer/newt/services.scm (run-networking-cbt-page): Remove
'network-management?' parameter and select only 'networking services.
(run-network-management-page): New procedure.
(run-services-page): Call it when DESKTOP is the empty list.

gnu/installer/newt/services.scm

@@ -45,19 +45,14 @@ choose the one to use on the log-in screen.")
       (condition
        (&installer-step-abort))))))
 
-(define (run-networking-cbt-page network-management?)
-  "Run a page allowing the user to select networking services.  When
-NETWORK-MANAGEMENT? is true, include network management services like
-NetworkManager."
+(define (run-networking-cbt-page)
+  "Run a page allowing the user to select networking services."
   (run-checkbox-tree-page
    #:info-text (G_ "You can now select networking services to run on your \
 system.")
    #:title (G_ "Network service")
-   #:items (filter (let ((types (if network-management?
-                                    '(network-management networking)
-                                    '(networking))))
-                     (lambda (service)
-                       (memq (system-service-type service) types)))
+   #:items (filter (lambda (service)
+                     (eq? 'networking (system-service-type service)))
                    %system-services)
    #:item->text (compose G_ system-service-name)
    #:checkbox-tree-height 5
@@ -67,9 +62,35 @@ system.")
       (condition
        (&installer-step-abort))))))
 
+(define (run-network-management-page)
+  "Run a page to select among several network management methods."
+  (let ((title (G_ "Network management")))
+    (run-listbox-selection-page
+     #:title title
+     #:info-text (G_ "Choose the method to manage network connections.
+
+We recommend NetworkManager or Connman for a WiFi-capable laptop; the DHCP \
+client may be enough for a server.")
+     #:info-textbox-width 70
+     #:listbox-items (filter (lambda (service)
+                               (eq? 'network-management
+                                    (system-service-type service)))
+                             %system-services)
+     #:listbox-item->text (compose G_ system-service-name)
+     #:sort-listbox-items? #f
+     #:button-text (G_ "Exit")
+     #:button-callback-procedure
+     (lambda _
+       (raise
+        (condition
+         (&installer-step-abort)))))))
+
 (define (run-services-page)
   (let ((desktop (run-desktop-environments-cbt-page)))
     ;; When the user did not select any desktop services, and thus didn't get
     ;; '%desktop-services', offer network management services.
     (append desktop
-            (run-networking-cbt-page (null? desktop)))))
+            (run-networking-cbt-page)
+            (if (null? desktop)
+                (list (run-network-management-page))
+                '()))))