More dir-per-part issues

1) super files calculate the common shared directory and set the file descriptor to use it, and set parts overrite to be the distinct tail.
The introduction of the dir-per-part flag, caused super part filenames to have the part-per-part subdir added twice.
Fix: since it is precalculated (via overridename) in super case, deliberately turn off flag when fetching the super file descriptor

2) Deseriazliaing a IFileDescriptor from xml was picking up the new file flags (which determines dir-per-part) from the wrong place.
Fix: pick up from Attr/