Cleanup of vtkMultiThreader, notably removed ancient HP & SGI options
- removed VTK_HP_PTHREADS & VTK_USE_SPROC options
- Stop #including vtkMultiThreader.h in some places it's not used.
- changed a few int to vtkTypeBool
- mark vtkThreadMessager as deprecated, nothing in VTK seems to use it
- fixed some typos