nixos/nix-daemon: support machine protocol

                The hostname of the build machine.

              '';

            };

            protocol = mkOption {

              type = types.enum [ "ssh" "ssh-ng" ];

              default = "ssh";

              example = "ssh-ng";

              description = lib.mdDoc ''

                The protocol used for communicating with the build machine.

                Use `ssh-ng` if your remote builder and your

                local Nix version support that improved protocol.

              '';

            };

            system = mkOption {

              type = types.nullOr types.str;

              default = null;

        concatMapStrings

          (machine:

            (concatStringsSep " " ([

              "${optionalString (machine.sshUser != null) "${machine.sshUser}@"}${machine.hostName}"

              "${machine.protocol}://${optionalString (machine.sshUser != null) "${machine.sshUser}@"}${machine.hostName}"

              (if machine.system != null then machine.system else if machine.systems != [ ] then concatStringsSep "," machine.systems else "-")

              (if machine.sshKey != null then machine.sshKey else "-")

              (toString machine.maxJobs)