Enhancement: Support restoring ownership by name on UNIX systems

Restic restore used to restore file ownership on UNIX systems by UID and GID.
It now allows restoring ownership by user and group name with
`--ownership-by-name`, so snapshots can be restored on systems where numeric IDs
do not match the backup host. POSIX ACLs are still restored by numeric value;
this change does not add ACL-by-name support.

https://github.com/restic/restic/issues/3572
https://github.com/restic/restic/pull/5449
