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)

src/sampling/surface/isoSurface/isoSurfacePoint.C

@@ -1362,7 +1362,7 @@ Foam::isoSurfacePoint::isoSurfacePoint
         (
             "C",
             fvmesh.pointsInstance(),
-            fvmesh.meshSubDir,
+            polyMesh::meshSubDir,
             fvmesh,
             IOobject::NO_READ,
             IOobject::NO_WRITE,