STYLE: adjust const access for linked-list iterators 'operator*'

- provides const/non-const access to the underlying list, but the
  iterator access itself is const.

- provide linked-list iterator 'found()' method for symmetry with
  hash-table iterators. Use nullptr for more clarity.

applications/test/DLList/Test-DLList.C

@@ -56,7 +56,6 @@ int main(int argc, char *argv[])
         Info<< "element:" << *iter << endl;
     }
 
-
     Info<< nl << "And again using the same STL iterator: " << nl << endl;
 
     forAllIters(myList, iter)