Make PermissionStatusFormat public

implementations of INodeAttributeProvider are required to provide an implementation of getPermissionLong() method. Unfortunately, the long permission format is an encoding of the user, group and mode with each field converted to int using SerialNumberManager which is package protected.

Thus it would be nice to make the PermissionStatusFormat enum public (and also make the toLong() static method public) so that user specified implementations of INodeAttributeProvider may use it.

This would also make it more consistent with AclStatusFormat which I guess has been made public for the same reason.