libxfs: restructure to match kernel layout

libxfs: restructure to match kernel layout

The kernel now has a libxfs directory, so we need to restructure the
userspace libxfs source tree to match the kernel layout. This
involves changing the location of libxfs include files and how they
are linked into include/xfs for all the subdirectories to see them.

This is a bit convoluted as an initial conversion step - we move all
the libxfs headers files from include/ to libxfs/ and then add build
rules to symlink the files directly to include/xfs so everything
still "sees" the same header files. This changes the build
dependencies slightly, as libxfs now must be built after include
(unchanged) but before anything else (new) so that we have a
complete include/xfs directory. This also affects install and
packaging rules, as header install rules are now split across
include and libxfs Makefiles.

Signed-off-by: Dave Chinner <dchinner@redhat.com>