To avoid problems with client threads starting execution before DR's signal handlers are in place, we delay client thread execution until DR starts executing the app.
Implements a broadcast event variant of event_t to support waking all client threads at start time.
Adjusts the client.signal and client.thread tests to handle this new restriction.
Removes the erroneous call to signal_reinstate_handlers() in client threads.
Fixes #2335 (closed)