Enhancement: (alpha) Store deviceID only for hardlinks

Set `RESTIC_FEATURES=device-id-for-hardlinks` to enable this alpha feature.
The feature flag will be removed after repository format version 3 becomes
available or be replaced with a different solution.

When creating backups from a filesystem snapshot, for example created using
BTRFS subvolumes, the deviceID of the filesystem changes compared to previous
snapshots. This prevented restic from deduplicating the directory metadata of
a snapshot.

When this alpha feature is enabled, the deviceID is only stored for hardlinks,
which significantly reduces the metadata duplication for most backups.

https://github.com/restic/restic/pull/4006
