nixos/network-interfaces-systemd: fix DHCP setting

The networkd backend logic for setting DHCP= on an interface is bugged
and inconsistent with the scripted logic. Consider this simple NixOS
configuration:

    {
      networking.useNetworkd = true;
      networking.interfaces.eth0.wakeOnLan.enable = true;
    }

The default value of networking.useDHCP is true, so we expect our eth0
interface to have DHCP enabled. With the scripted backend, this works.
But the networkd backend generates the following 40-eth0.network file:

    [Match]
    Name=eth0

    [Network]
    DHCP=no
    IPv6PrivacyExtensions=kernel

This is happening because the wakeOnLan configuration creates a key in
networking.interfaces, and the networkd backend erroneously checks that
instead of for explicitly configured IP addresses as in the scripted
backend. The documentation is also inconsistent across various options.

This change aligns the networkd backend and option documentation to the
actual behavior of the scripted backend, and updates a test to account
for this behavior for both backends.

nixos/modules/tasks/network-interfaces-systemd.nix

@@ -92,7 +92,7 @@ let
     networks."40-${i.name}" = mkMerge [ (genericNetwork id) {
       name = mkDefault i.name;
       DHCP = mkForce (dhcpStr
-        (if i.useDHCP != null then i.useDHCP else false));
+        (if i.useDHCP != null then i.useDHCP else (config.networking.useDHCP && i.ipv4.addresses == [ ])));
       address = forEach (interfaceIps i)
         (ip: "${ip.address}/${toString ip.prefixLength}");
       routes = forEach (interfaceRoutes i)