- partial revert for 13740de4 (#2158)

  MS-MPI does not currently have a MPI_Comm_create_group(),
  so keep using MPI_Comm_create() there.

  Only affects multi-world simulations.

CONFIG: retain dummy version of libPstream.dll

- retain as libPstream.dll-dummy so that it is available for
  manual replacement of the regular libPstream.dll (#2290)

  Keep extra copy of libPstream.dll as libPstream.dll-msmpi
  (for example) for manual replacement.