xfs: create libxfs infrastructure

To minimise the differences between kernel and userspace code,
split the kernel code into the same structure as the userspace code.
That is, the gneric core functionality of XFS is moved to a libxfs/
directory and treat it as a layering barrier in the XFS code.

This patch introduces the libxfs directory, the build infrastructure
and an initial source and header file to build. The libxfs directory
will contain the header files that are needed to build libxfs - most
of userspace does not care about the location of these header files
as they are accessed indirectly. Hence keeping them inside libxfs
makes it easy to track the changes and script the sync process as
the directory structure will be identical.

To allow this changeover to occur in the kernel code, there are some
temporary infrastructure in the makefiles to grab the header
filesystem from both locations. Once all the files are moved,
modifications will be made in the source code that will make the
need for these include directives go away.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>