Enhancement: Enable file system privileges on Windows before access

Restic attempted to enable Windows file system privileges when
reading or writing security descriptors - after potentially being wholly
denied access to previous items. It also read file extended attributes without
using the privilege, possibly missing them and producing errors.

Restic now attempts to enable all file system privileges before any file
access. It also requests extended attribute reads use the backup privilege.

https://github.com/restic/restic/pull/5424