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/lte-uesoftmodem.c

@@ -699,7 +699,7 @@ int main( int argc, char **argv ) {
   printf("TYPE <CTRL-C> TO TERMINATE\n");
   //getchar();
   printf("Entering ITTI signals handler\n");
-  itti_wait_tasks_end();
+  itti_wait_tasks_end(NULL);
   printf("Returned from ITTI signal handler\n");
   oai_exit=1;
   printf("oai_exit=%d\n",oai_exit);