libxfs: directly include libxfs headers

libxfs: directly include libxfs headers

As a step closer to the kernel code, have the libxfs code explicitly
include the headers they require to compile. This causes lots of
churn in the include files but starts to bring sanity to current
include mess.

IOWs, this is the first step towards libxfs having a clean
internal/external API demarkation for userspace. The internal libxfs
build is controlled through libxfs/xfs.h, and that defines the
functions that are exported by the library via the libxfs_*
namespace. This also starts moving the internal header structure to
the same layout and dependencies as the kernel code so that we can
separate out include/libxfs.h so that it only needs to include other
header files and doesn't ave to provide lots of "work around kernel
oddities" and export function prototypes that the internal libxfs
code does not define prototypes for.

There's still lots of follow patches to make this a reality, but
this is the first major step in cleaning up the mess.

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