+S4U:
+ - Barrier::wait returns SG_BARRIER_SERIAL_THREAD for (only) one actor
+ for consistency with pthread_barrier_wait()
+ - Host::get_englobing_zone() returns the englobing netzone
+ - Actor::on_destruction is now called in the destructor
+ Actor::on_termination new signal called when the actor terminates
+ its code.
+
+MSG:
+ - convert a new set of functions to the S4U C interface and move the old MSG
+ versions to legacy (MSG_process_self*, MSG_process_{un}ref, ...)
+