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/mesh/manipulation/setsToZones/setsToZones.C

@@ -86,7 +86,7 @@ int main(int argc, char *argv[])
 
     #include "createNamedPolyMesh.H"
 
-    const fileName setsSubPath(mesh.dbDir()/polyMesh::meshSubDir/"sets");
+    const fileName setsSubPath(mesh.meshDir()/"sets");
 
     // Search for list of objects for the time of the mesh
     word setsInstance = runTime.findInstance