nixos/networkd: reimplement useDHCP in a sensible way

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

@@ -43,12 +43,6 @@ in
     } {
       assertion = cfg.defaultGateway6 == null || cfg.defaultGateway6.interface == null;
       message = "networking.defaultGateway6.interface is not supported by networkd.";
-    } {
-      assertion = cfg.useDHCP == false;
-      message = ''
-        networking.useDHCP is not supported by networkd.
-        Please use per interface configuration and set the global option to false.
-      '';
     } ] ++ flip mapAttrsToList cfg.bridges (n: { rstp, ... }: {
       assertion = !rstp;
       message = "networking.bridges.${n}.rstp is not supported by networkd.";
@@ -92,6 +86,40 @@ in
             };
           };
         });
+        networks."99-ethernet-default-dhcp" = lib.mkIf cfg.useDHCP {
+          # We want to match physical ethernet interfaces as commonly
+          # found on laptops, desktops and servers, to provide an
+          # "out-of-the-box" setup that works for common cases.  This
+          # heuristic isn't perfect (it could match interfaces with
+          # custom names that _happen_ to start with en or eth), but
+          # should be good enough to make the common case easy and can
+          # be overridden on a case-by-case basis using
+          # higher-priority networks or by disabling useDHCP.
+
+          # Type=ether matches veth interfaces as well, and this is
+          # more likely to result in interfaces being configured to
+          # use DHCP when they shouldn't.
+
+          # We set RequiredForOnline to false, because it's fairly
+          # common for such devices to have multiple interfaces and
+          # only one of them to be connected (e.g. a laptop with
+          # ethernet and WiFi interfaces). Maybe one day networkd will
+          # support "any"-style RequiredForOnline...
+          matchConfig.Name = ["en*" "eth*"];
+          DHCP = "yes";
+          linkConfig.RequiredForOnline = lib.mkDefault false;
+        };
+        networks."99-wireless-client-dhcp" = lib.mkIf cfg.useDHCP {
+          # Like above, but this is much more likely to be correct.
+          matchConfig.WLANInterfaceType = "station";
+          DHCP = "yes";
+          linkConfig.RequiredForOnline = lib.mkDefault false;
+          # We also set the route metric to one more than the default
+          # of 1024, so that Ethernet is preferred if both are
+          # available.
+          dhcpV4Config.RouteMetric = 1025;
+          ipv6AcceptRAConfig.RouteMetric = 1025;
+        };
         networks."40-${i.name}" = mkMerge [ (genericNetwork id) {
           name = mkDefault i.name;
           DHCP = mkForce (dhcpStr