libxfs: disambiguate xfs.h

libxfs: disambiguate xfs.h

There are two "xfs.h" header files in xfsprogs. include/xfs.h
contains the userspace API definitions for the platform and ioctl
interfaces. libxfs/xfs.h contains support infrastructure
to allow kernel code to compile in userspace. They have different
purposes, and so we need to disambiguate them so it is clear what
header files are being included in which files.

We can't change include/xfs.h as it is exported and installed into
/usr/include/xfs, and that means we have to rename the libxfs
internal header file. Rename this to "libxfs_priv.h" so it is clear
that it is private to libxfs, and update all the libxfs code to
include it.

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