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).

common/utils/ocp_itti/intertask_interface.h

@@ -550,10 +550,13 @@ MessageDef *itti_alloc_new_message_sized(
   MessagesIds       message_id,
   MessageHeaderSize size);
 
-/** \brief handle signals and wait for all threads to join when the process complete.
-   This function should be called from the main thread after having created all ITTI tasks.
+/** \brief Wait for SIGINT/SIGTERM signals to unblock ITTI.
+   This function should be called from the main thread after having created all ITTI tasks. If handler is NULL, a default handler is installed.
+    \param handler a custom signal handler. To unblock, it should call itti_wait_tasks_unblock()
  **/
-void itti_wait_tasks_end(void);
+void itti_wait_tasks_end(void (*handler)(int));
+/** \brif unblocks ITTI waiting in itti_wait_tasks_end(). **/
+void itti_wait_tasks_unblock(void);
 void itti_set_task_real_time(task_id_t task_id);
 
 /** \brief Send a termination message to all tasks.