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/preProcessing/changeDictionary/changeDictionary.C

@@ -540,7 +540,7 @@ int main(int argc, char *argv[])
                 "boundary",
                 runTime.findInstance
                 (
-                    polyMesh::regionName(regionName)/polyMesh::meshSubDir,
+                    polyMesh::meshDir(regionName),
                     "boundary",
                     IOobject::READ_IF_PRESENT
                 ),