xfsprogs: move custom interface definitions out of xfs_fs.h

xfsprogs: move custom interface definitions out of xfs_fs.h

There are several definitions and structures present in the userspace
copy of libxfs/xfs_fs.h which support older, custom xfs interfaces
which are now common definitions in the vfs.

Move them into their own compat header to minimize the shared file
differences.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>