systemd-unit-options: startAt can be a list

[Profpatsch]

OnCalendar entrys can be specified multiple times in a systemd timer, to
make more complex scheduling possible.

Tested by manually checking the timer generated by the following:

systemd = {
  services.huhu = {
    description = "meh";
    wantedBy = [ "default.target" ];
    serviceConfig.ExecStart = "/bin/sh -c 'printf HUHU!'";
    startAt = [ "*:*:0/30" "*:0/1:15" ];
  };
};

It prints HUHU to the log at seconds 0, 15 and 30 of each minute.

[mention-bot]

@Profpatsch, thanks for your PR! By analyzing the annotation information on this pull request, we identified @edolstra, @bluescreen303 and @rickynils to be potential reviewers