Only the `Sys()` value from os.FileInfo is kept as field `sys` to
support Windows. The os.FileInfo removal ensures that for values like
`ModTime` that existed in both data structures there's no more confusion
which value is actually used.
The actual implementation still relies on file paths, but with the
abstraction layer in place, an FS implementation can ensure atomic file
accesses in the future.
On FreeBSD, limited users may not be able to even list xattrs for the
parent directories above the snapshot source paths. As this can cause
the backup to fail, just ignore those errors.
