fix frame dynamic linkage in the face of partial continuation application

fix frame dynamic linkage in the face of partial continuation application

* libguile/vm-i-system.c (new-frame): Though it was appealing to set the
  dynamic link here on the incomplete frame, we no longer do that, for
  the reasons mentioned in the code.
  (call, mv-call): Adapt to set the frame's dynamic link.

* libguile/vm-engine.c (vm_engine): Don't set dynamic link here, even
  for boot program.

* libguile/frames.c (scm_frame_num_locals, scm_frame_local_ref)
  (scm_frame_local_set_x): Fix up not-yet-active frame detection.