in debug mode, make sure that calls to the vm can be captured via make-stack

* libguile/vm-engine.c (VM_PUSH_DEBUG_FRAMES): New knob, if true we much
  with the scm_i_last_debug_frame when entering the VM, because sometimes
  the evaluator doesn't do it for us.
  (VM_ENGINE): Plug through debug frame fondling. Now, program exit comes
  back to the main text. Rename err_args to finish_args, and reuse for
  the return value.

* libguile/vm-engine.h (PUSH_LIST):
* libguile/vm-i-loader.c:
* libguile/vm-i-scheme.c:
* libguile/vm-i-system.c: Update for finish_args.
  (halt): goto vm_done, now, instead of returning directly.

libguile/vm-engine.h

@@ -294,7 +294,7 @@ do						\
   for (; scm_is_pair (l); l = SCM_CDR (l))      \
     PUSH (SCM_CAR (l));                         \
   if (SCM_UNLIKELY (!NILP (l))) {               \
-    err_args = scm_list_1 (l);                  \
+    finish_args = scm_list_1 (l);               \
     goto vm_error_improper_list;                \
   }                                             \
 } while (0)