pacreport: add systemd service and timer

[davidfields]

Hi,

I think it would be useful to include a systemd service that runs pacreport and a systemd timer that activates the service.

For pacutils users, using the timer and service would of course be entirely optional as neither would be enabled by default.

Additionally (or alternatively), the same could be done for pacsync and/or paccheck.

I would be happy to write the systemd unit files.

Thanks!