nixos/networking: allow users to override FQDN (#391522)

    };

    networking.fqdn = mkOption {

      readOnly = true;

      type = types.str;

      default =

        if (cfg.hostName != "" && cfg.domain != null) then

          "${cfg.hostName}.${cfg.domain}"

        else

          throw ''

            The FQDN is required but cannot be determined. Please make sure that

            both networking.hostName and networking.domain are set properly.

            The FQDN is required but cannot be determined from `networking.hostName`

            and `networking.domain`. Please ensure these options are set properly or

            set `networking.fqdn` directly.

          '';

      defaultText = literalExpression ''"''${networking.hostName}.''${networking.domain}"'';

      description = ''

        The fully qualified domain name (FQDN) of this host. It is the result

        of combining `networking.hostName` and `networking.domain.` Using this

        option will result in an evaluation error if the hostname is empty or

        The fully qualified domain name (FQDN) of this host. By default, it is

        the result of combining `networking.hostName` and `networking.domain.`

        Using this option will result in an evaluation error if the hostname is empty or

        no domain is specified.

        Modules that accept a mere `networking.hostName` but prefer a fully qualified