Refactor handling of active VM registers

* libguile/threads.h (scm_thread): Remove unused jmp_buf regs member.
* libguile/vm.h (struct scm_vm): Rename resumable_prompt_cookie to just
  "registers"; we know it's a jmp_buf pointer.
* libguile/vm.c (scm_call_n):
* libguile/throw.c (catch):
* libguile/eval.c (eval):
* libguile/control.c (scm_suspendable_continuation_p): Adapt to cookie
  renaming.

libguile/vm.h

@@ -20,6 +20,8 @@
 #ifndef _SCM_VM_H_
 #define _SCM_VM_H_
 
+#include <setjmp.h>
+
 #include <libguile/gc.h>
 #include <libguile/programs.h>
 
@@ -56,7 +58,7 @@ struct scm_vm {
   union scm_vm_stack_element *stack_top; /* highest address in allocated stack */
   SCM overflow_handler_stack;   /* alist of max-stack-size -> thunk */
   SCM hooks[SCM_VM_NUM_HOOKS];	/* hooks */
-  const void *resumable_prompt_cookie; /* opaque cookie */
+  jmp_buf *registers;           /* registers captured at latest vm entry  */
   int engine;                   /* which vm engine we're using */
 };