STYLE: make polyMesh constructor explicit

- use polyMesh::defaultRegion instead of fvMesh::defaultRegion and
  others via static inheritance. (92 vs ~25 occurrences)

applications/utilities/parallelProcessing/redistributePar/redistributePar.C

@@ -2905,7 +2905,7 @@ int main(int argc, char *argv[])
             const word& regionName = regionNames[regioni];
             const fileName meshSubDir
             (
-                regionName == fvMesh::defaultRegion
+                regionName == polyMesh::defaultRegion
               ? fileName(polyMesh::meshSubDir)
               : regionNames[regioni]/polyMesh::meshSubDir
             );