Add support for backing up and restoring extended attributes

Security Enhanced Linux (SELinux) enabled systems make extensive use of extended attributes. In addition to the standard user, group, and permission, each file has an associated SELinux context stored as an extended attribute. This context is used to define which operations a given program is permitted to perform on that file. Storing contexts on an SELinux system is as critical as storing ownership and permissions. In the case of a full system restore, the system will not even be able to boot until all critical system files have been properly relabeled.