backuptar.WriteTarFileFromBackupStream writes tarballs with 0 permission bits

[TBBle]

I noticed during microsoft/hcsshim#901 that when go-winio creates a tar stream from a backup, it never sets the permission bits of the file mode.

I'm not sure if this is a problem. It's probably surprising when the tarballs are used with non-Windows OSes, and I suspect I'll find a workaround for this in, e.g., BuildKit's non-Windows-platform WCOW layer manipulation code, if I go looking.

Base layer WCOW tarballs from MS for Windows do have permissions on files in the tarball, except a couple of top-level directories (Files/, UtilityVM/Files/, and their parents). I haven't checked non-base layers from MS to see if they have the same behaviour as base layers (normal default permissions), or as go-winio (no permissions).

It would be a fairly simple change to backuptar.BasicInfoHeader to implement a default file mode of 0644 and default directory mode of 0755, but I'm not sure if that would have other repercussions or would otherwise be undesirable.