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.