[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.
@jerryk55 Would it be less horrible to make 16 a constant?  It looks like it's also hardcoded in line 87 above.