* dcrt0.cc (do_exit): Don't set ES_SET_MUTO here. Call get_exit_lock()

* dcrt0.cc (do_exit): Don't set ES_SET_MUTO here.  Call get_exit_lock()
instead.
(get_exit_lock): New function.  Grabs the lock and sets initial exit_state.
* exceptions.cc (try_to_debug): Use low_priority_sleep.
(sigpacket::process): Avoid handler if this is an exec stub.
(signal_exit): Use get_exit_lock rather than manipulating the exit_lock
critical section directly.
* pinfo.cc (pinfo::exit): Ditto.
* winsup.h (get_exit_lock): Declare.
(exit_lock): Delete declaration.