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/parallelProcessing/decomposePar/decomposePar.C

@@ -813,11 +813,7 @@ int main(int argc, char *argv[])
                     IOobject io
                     (
                         "faBoundary",
-                        mesh.time().findInstance
-                        (
-                            mesh.dbDir()/polyMesh::meshSubDir,
-                            "boundary"
-                        ),
+                        mesh.time().findInstance(mesh.meshDir(), "boundary"),
                         faMesh::meshSubDir,
                         mesh,
                         IOobject::READ_IF_PRESENT,