Enhancement: Support restoring ownership by name on UNIX systems

The `restore` command used to restore file ownership on UNIX systems by UID
and GID. It now supports restoring ownership by user and group name with
`--ownership-by-name`, so that snapshots can be restored on systems where
numeric IDs do not match those on the backup host.

Note: 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
