smartd: change 'devices' option from "list of strings" to "list of attribute sets"

The smartd used to expect a list of devices to monitor. After this patch, it
expects a list of attribute sets, which may have two attributes:

 - device: path to the device (required)
 - options: smartd options to apply to this particular device (optional)

A concrete example configuration would be:

  services.smartd = {
    enable = true;
    devices = [ { device = "/dev/sda"; } { device = "/dev/sdb"; options = "-d sat"; } ];
  };

Furthermore, the config option 'deviceOpts' can be used to configure options
that are applied to *every* device.