ENH: minor simplifications for handling region meshes

- static version of polyMesh::meshDir(), which takes a region name

      polyMesh::meshDir(regionName)
  vs
      polyMesh::regionName(regionName)/polyMesh::meshSubDir

STYLE: use polyMesh::regionName(..) instead of comparing to defaultRegion

STYLE: use getOrDefault when retrieving various -region options

FIX: polyMesh::dbDir() now checks registry name, not full path (#3033)

applications/utilities/postProcessing/dataConversion/foamToTetDualMesh/foamToTetDualMesh.C

@@ -184,7 +184,7 @@ int main(int argc, char *argv[])
         (
             "pointDualAddressing",
             tetDualMesh.facesInstance(),
-            tetDualMesh.meshSubDir,
+            polyMesh::meshSubDir,
             tetDualMesh,
             IOobject::MUST_READ
         )