Bugfix: Ignore EOPNOTSUPP as an error for xattr

Restic 0.18.0 added xattr support for NetBSD 10+, but not all NetBSD
filesystems support xattrs.  Other BSD systems can likewise return
EOPNOTSUPP, so restic now simply ignores EOPNOTSUPP errors for xattrs.

https://github.com/restic/restic/issues/5344