Add snapper-sync to synchronize the highest snapshot number

[jameslai-dev]

To resolve the potential snapshot number collision after a system restore, as mentioned in #1105 (comment), this pull request adds a snapper-sync script to synchronize the highest snapshot number between the source and the backup target. The script can be triggered automatically by enabling snapper-sync.service.

Detailed changes:

• Added script/snapper-sync: A script to synchronize the highest snapshot number across snapperd and snbk. This script does nothing if snbk is not installed. The script requires both snapper and snbk to be installed.
• Added data/snapper-sync.service: An optional systemd service to trigger snapper-sync automatically.
• Changed data/boot.timer and data/timeline.timer to optionally depend on snapper-sync.service.
• Changed snapper.spec.in and dist/debian to include snapper-sync related files in the snapper snapper-backup package.
• Added documentation for snapper-sync in the snapper snapper-backup manual.

[aschnell]

Please move the new files to the snapper-backup package. Only when that is installed the new service makes sense.

[jameslai-dev]

Thanks for your feedback. The snapper-sync files have been moved to the Snapper Backup package. (I thought it would be weird to make snapper-boot.timer and snapper-timeline.timer depend on a non-existent service, but it seems harmless after testing.)

A few minor changes have been made:

• Changed snapper-sync to require the snbk binary.
• Adjusted snapper-sync documentation and moved it to /doc/snbk.xml.in

Please take a look!

[jameslai-dev]

I just realized there is a drop-in mechanism for overriding an existing service, but I haven't used it before: https://wiki.archlinux.org/title/Systemd#Drop-in_files

Would it be appropriate to include drop-in files in Snapper Backup to override snapper-boot.timer and snapper-timeline.timer?
If this is possible, the Snapper timer units can remain unmodified.

[aschnell]

Yes, drop-in overrides look like a good idea.