Reimplement itti_wait_tasks_end() to allow custom wait handlers

In the UE, we wish to handle ^C in the UE to send a message before
terminating, so we allow custom wait handlers to implement such
functionality. Use itti_wait_tasks_unblock() to unblock
itti_wait_tasks_end().

Also, implement itti_wait_tasks_end() using semaphores, as signals are
not async-signal-safe (and therefore should not be used in a signal
handler).

executables/nr-cuup.c

@@ -153,7 +153,7 @@ int main(int argc, char **argv)
   itti_send_msg_to_task(TASK_CUUP_E1, 0, msg);
 
   printf("TYPE <CTRL-C> TO TERMINATE\n");
-  itti_wait_tasks_end();
+  itti_wait_tasks_end(NULL);
 
   logClean();
   printf("Bye.\n");