Adding SugarExt for setting hidden attribute on FAT files

This extension is used by Sugar to set the HIDDEN attribute for
.Sugar-Metadata. By creating an extension, we avoid the need to add an
additional dependency (fatattr) to Sugar. The code is modeled after
fatattr, using an IOCTL call get and set FAT file attributes.

There is a corresponding patch for model.py in sugar/src/jarabe/journal