libxfs: do all xfs->libxfs defines inside libxfs/

libxfs: do all xfs->libxfs defines inside libxfs/

Currently include/libxfs does some #define libxfs.... xfs...
conversions, allowing external functions to call xfs namespace
functions directly. All of the exported functions from libxfs shoul
dbe compiled in as libxfs_... namespace symbols, and so such defines
should be in libxfs/xfs.h and done the opposite way around.

This exposes an awful lot of incorrectly namespaced calls in the
userspace code and highlights functions that we weren't expressly
exporting from libxfs, so we need to fix them up at the same time.

As such, this patchset starts laying the groundwork for a more
formalised libxfs interface definition. The libxfs code will need
to be restructured to match the kernel code restructing that was
done in 3.17, so this interface will become more formalised and
defined as that work is done.

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