fix a prompt bug

fix a prompt bug

* libguile/control.h:
* libguile/control.c (scm_c_make_prompt): Instead of taking a VM arg,
  take the registers directly.
  (scm_c_abort): Declare as returning void. In fact it will never
  return.

* libguile/eval.c (eval):
* libguile/throw.c (pre_init_catch): Adapt to prompt API change.

* libguile/vm-i-system.c (prompt): Pass the abort ip as the ip to
  scm_c_make_prompt. This fixes a bug in which we used the "offset"
  local var, but it wasn't guaranteed to be around after a longjmp.