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-i-scheme.c

@@ -136,7 +136,7 @@ VM_DEFINE_FUNCTION (90, cons, "cons", 2)
 
 #define VM_VALIDATE_CONS(x)                     \
   if (SCM_UNLIKELY (!scm_is_pair (x)))          \
-    { err_args = x;                             \
+    { finish_args = x;                          \
       goto vm_error_not_a_pair;                 \
     }