Only track device id if it is a hardlink (has linkcount > 1)

[Valentin-Metz]

Rustic currently tracks the device id by default. This causes issues when working with removable drives or ZFS.
Specifically, the device id is not stable in such a case, which leads to a lot of unnecessarily changed blobs.

There has been an extensive discussion of this in restic/restic#3041 and the proposed fix restic/restic#4006 is to only track the device id when necessary. This would provide a sane default instead of manually having to supply --ignore-devid, which might have unintended side-effects on files with hardlinks.