koreader: Add service unit

[matteodelabre]

This PR adds a systemd service unit for starting KOReader at boot time (see #437). If KOReader fails to start, it falls back to Xochitl. It also updates the Xochitl service definition to alias to launcher.service; this is to prevent situations where both KOReader and Xochitl are enabled to start at boot time (see #438).

[Eeems]

The package should also have install/uninstall scripts handling letting the user know about the service, and properly stopping/disabling it on uninstall.

[matteodelabre]

Good point, thanks! I added that.

[Eeems]

Changes look good, but I haven't tested.

[matteodelabre]

Thanks for approving the PR—I had to push a new commit to fix the formatting, which auto-dismissed your review.

Changes look good, but I haven't tested.

I’ve performed the following tests on my rM2: checked that enabling the service does start KOReader at boot, that it’s not possible to enable both KOReader and Xochitl at the same time, and that the configure() script suggests the appropriate command for enabling KOReader, depending on whether Xochitl is enabled.

I don’t think there is any rM1-specific things in this PR, so it should work fine on rM1.

[Eeems]

It looks like while you can't enable both tarnish and koreader with this, you can start them both at the same time. Is that expected?

[matteodelabre]

Yes, this is unfortunate, but I didn’t find a clean way to prevent this. This is not a regression from the Conflicts=-based approach though, since even conflicting services can be manually started at the same time.