xfsprogs: use common code for multi-disk detection

xfsprogs: use common code for multi-disk detection

Both xfs_repair and mkfs.xfs need to agree on what is a "multidisk:
configuration - mkfs for determining the AG count of the filesystem,
repair for determining how to automatically parallelise it's
execution. This requires a bunch of common defines that both mkfs
and reapir need to share.

In fact, most of the defines in xfs_mkfs.h could be shared with
other programs (i.e. all the defaults mkfs uses) and so it is
simplest to move xfs_mkfs.h to the shared include directory and add
the new defines to it directly.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Jan Tulak <jtulak@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>