WIP: full XATTR support.  This commit refactors the top-level MS file
API handlers into separate files, and refactors the common request
processing logic.  The end-result is much more maintainable and
extensible, with much simpler methods.  In addition, this commit
includes some bugfixes discovered in testing the refactoring with
respect to how file operations interact with the cache.

There is still a known regression in the code-base relating to how two
UGs share writes with one another.  This is still a WIP, and will be
fixed in my next commit.