(_pth_strerror): Renamed to ...
(w32_strerror): .. this. And let callers provide a buffer.
(spawn_helper_thread): Removed HD arg and hardwire the stack size
to 32k.
(do_pth_wait): Removed use of ATTR; not needed for the helper
threads.
(helper_thread): Renamed to ..
(launch_thread): .. this. Release handle if not joinable.
(struct pth_priv_hd_s): Renamed to ...
(struct thread_info_s): .. this. Add member JOINABLE and TH.