openfoam: consistent boundary point order

- in OpenFOAM-1.5 and earlier, the patch meshPoints were in increasing
  order but this gave problems in processor point synchronisation and
  was changed to use the order in which faces are visited.
  The VTK reader continued to use older convention (sorted order).

  This was not a bug per se, since the discrepancy in the local patch
  point ordering did not actually affect any of the handled field
  types, which invariably used the mesh point indices to retrieve
  values from the internal field.

  The recently added support for point patch "value" fields exposed
  this inconsistency and prompted this change.