dhcpcd service: always want by multi-user

When default gateways were specified the service wasn't wanted by anything at
all before.

nixos/modules/services/networking/dhcpcd.nix

···
     in
       { description = "DHCP Client";
 
-        wantedBy = optional (!hasDefaultGatewaySet) "network-online.target";
+        wantedBy = [ "multi-user.target" ] ++ optional (!hasDefaultGatewaySet) "network-online.target";
         after = [ "network.target" ];
         wants = [ "network.target" ];