- Verify that file holes work (they should, but must be checked).  Add a
  regression test for this feature.

- Fix and complete code marked with `XXX' and `TODO' tags.

- Adjust code style - remove the /* ---- */ markers and use standard
  struct vop_*_args assignment into *ap with comment like:
        struct vop_link_args /* {
                struct vnode *a_dvp;
                struct vnode *a_vp;
                struct componentname *a_cnp;
        } */ *ap = v;

  jmmv: I may agree with the removal of the visual lines, but not with the
  vop_*_args assignment.  Those are very ugly and add a lot of redundancy.