Previously, for a machine that only has wired networking, and only one
such network, we'd have to go through two selection boxes. Now we just
skip both.
* gnu/installer/newt/ethernet.scm (run-ethernet-page): When
'ethernet-services' returns one element, return it directly without
opening a listbox selection.
* gnu/installer/newt/network.scm (run-technology-page): Likewise.