REGRESSION: overly aggressive handling of MPI groups

- attempted reduction in bookkeeping (commit: 068ab8ccc7) meant that
  the worldComm didn't have a group from which sub-communicators could
  be spun off.

- do not force reset of PstreamBuffers positions

STYLE: UPstream::globalComm instead of '0'

src/functionObjects/utilities/syncObjects/syncObjects.C

@@ -90,9 +90,8 @@ void Foam::functionObjects::syncObjects::sync()
     // Note provision of explicit all-world communicator
     PstreamBuffers pBufs
     (
-        Pstream::commsTypes::nonBlocking,
-        UPstream::msgType(),
-        0
+        UPstream::globalComm,
+        UPstream::commsTypes::nonBlocking
     );