rewinding prompts works

* libguile/control.h (SCM_PROMPT_HANDLER): Remove, it was unused.
  (SCM_PROMPT_DYNWINDS): Rename from SCM_PROMPT_DYNENV.

* libguile/control.c: (scm_c_make_prompt): Take another arg, the winds
  that are to be in place for the prompt. Fix allocation to be 4 words
  instead of 5 (the handler was never used).

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

* libguile/vm-i-system.c (partial-cont-call): Grovel the new elements of
  the wind list in order to call setjmp() on the new prompts. Pass
  cookie to vm_reinstate_partial_continuation.
  (prompt): Adapt to scm_c_make_prompt change.

* libguile/vm.c (vm_reinstate_partial_continuation): Take a cookie arg,
  used when winding captured prompts onto the stack. Winding a prompt
  implies making a new prompt, actually -- with new registers, a new
  jump buffer, new winds, etc.

* test-suite/tests/control.test ("rewinding prompts"): Add a test for
  rewinding prompts.

test-suite/tests/control.test

@@ -204,3 +204,14 @@
       (equal? (k) 1))
     (pass-if "post"
       (equal? (fluid-ref fl) 0))))
+
+(with-test-prefix "rewinding prompts"
+  (pass-if "nested prompts"
+    (let ((k (% 'a
+                (% 'b
+                   (begin
+                     (abort 'a)
+                     (abort 'b #t))
+                   (lambda (k x) x))
+                (lambda (k) k))))
+      (k))))