change throw mechanism to fit better with cleanup mechanism:

- balls are now stored off line and recovered by Prolog code when everything
is safe.
- if a ball exists, throw uses the *existing* one (be careful not to have one
laying around).
- Jump routine cleans up every cp except ones for setup_call handling, backtracking simply calls setup handlers.

C/tracer.c

@@ -269,7 +269,7 @@ low_level_trace(yap_low_level_port port, PredEntry *pred, CELL *args)
       printf("\n");
  }
 #endif
-  fprintf(Yap_stderr,"%lld ",vsc_count);
+  fprintf(Yap_stderr,"%lld %p ", vsc_count, TR);
 #if defined(THREADS) || defined(YAPOR)
   fprintf(Yap_stderr,"(%d)", worker_id);
 #endif