[WIP] Add XFS Filesystem Support

The following portions of the XFS code have been (at least partially) tested:
1) Superblocks
2) Allocation Groups
3) Inodes
4) Directories, including:
5) ShortForm Directory Headers and Entries
6) Single Extent Directory Headers and Entries

There is additional code in this PR that is being checked in but has not yet
been validated.  Ignore for now please.

@roliveri @Fryguy @chessbyte please review at your convenience.
All DIR, Dir, and dir have been changed to DIRECTORY, Directory, and directory, respectively.  
EXCEPT - where a constant is named and valued exactly as in the 'C' code.  
E.G - XFS_DIR2_FD_COUNT