ENH: add UPstream::subProcs() static method

- returns a range of `int` values that can be iterated across.
  For example,

      for (const int proci : Pstream::subProcs()) { ... }

  instead of

      for
      (
          int proci = Pstream::firstSlave();
          proci <= Pstream::lastSlave();
          ++proci
      )
      {
          ...
      }

src/parallel/distributed/distributedTriSurfaceMesh/distributedTriSurfaceMesh.C

@@ -1655,7 +1655,7 @@ Foam::distributedTriSurfaceMesh::independentlyDistributedBbs
     bool masterOnly;
     {
         masterOnly = true;
-        for (label proci = 1; proci < Pstream::nProcs(); proci++)
+        for (const int proci : Pstream::subProcs())
         {
             if (triIndexer.localSize(proci) != 0)
             {