openfoam: adjust face/owner reading

- handling of really old-format caused inadvertent short-circuit.
  Also check neighbour for max cell id.

- preserve face point order when flipping.

  When faces are flipped (reversed) in OpenFOAM, the face point 0 is
  preserved to ensure a consistent and predictable walk order.
  Probably less important to preserve for VTK, but may help in future
  and doesn't add any overhead to so.

- some code simplication for inserting polyhedral cells.