Refactoring ugly code in MeshTools::find_nodal_neighbors().

* Variable names were ugly.
* Variables unnecessarily declared outside of loops.
* "empty while loop with post-decrement" is ridiculous.
* Formatting inconsistent with rest of libmesh.
* Did not use Elem interfaces effectively.

This patch should also be cherry-picked onto master, it doesn't have
to wait until the subdivision surfaces stuff is merged.