ENH: simplify handling of pushed/freed requests

- previously had an additional stack for freedRequests_,
  which were used to 'remember' locations into the list of
  outstandingRequests_ that were handled by 'waitRequest()'.

  This was principally done for sanity checks on shutdown,
  but we now just test for any outstanding requests that
  are *not* MPI_REQUEST_NULL instead (much simpler).

  The framework with freedRequests_ also had a provision to 'recycle'
  them by popping from that stack, but this is rather fragile since it
  would only triggered by some collectives
  (MPI_Iallreduce, MPI_Ialltoall, MPI_Igather, MPI_Iscatter)
  with no guarantee that these will all be properly removed again.
  There was also no pruning of extraneous indices.

ENH: consolidate internal reset/push of requests

- replace duplicate code with inline functions
  reset_request(), push_request()

ENH: null out trailing requests

- extra safety (paranoia) for the UPstream::Request versions
  of finishedRequests(), waitAnyRequest()

CONFIG: document nPollProcInterfaces in etc/controlDict

- still experimental, but at least make the keyword known

src/Pstream/mpi/UIPstreamRead.C

@@ -105,6 +105,8 @@ Foam::label Foam::UIPstream::read
     UPstream::Request* req
 )
 {
+    PstreamGlobals::reset_request(req);
+
     if (debug)
     {
         Pout<< "UIPstream::read : starting read from:" << fromProcNo
@@ -205,26 +207,18 @@ Foam::label Foam::UIPstream::read
             return 0;
         }
 
-        profilingPstream::addRequestTime();
-
         if (debug)
         {
             Pout<< "UIPstream::read : started read from:" << fromProcNo
                 << " tag:" << tag << " read size:" << label(bufSize)
                 << " commsType:" << UPstream::commsTypeNames[commsType]
-                <<
+                << " request:" <<
                 (req ? label(-1) : PstreamGlobals::outstandingRequests_.size())
                 << Foam::endl;
         }
 
-        if (req)
-        {
-            *req = UPstream::Request(request);
-        }
-        else
-        {
-            PstreamGlobals::outstandingRequests_.push_back(request);
-        }
+        PstreamGlobals::push_request(request, req);
+        profilingPstream::addRequestTime();
 
         // Assume the message is completely received.
         return bufSize;