* cygtls.h (exitsock): New element.

* cygtls.h (exitsock): New element.
(exitsock_sin): Ditto.
* cygtls.cc (_cygtls::init_thread): Initialize exitsock to invalid handle.
(_cygtls::call2): Close exitsock if it is valid.
* select.cc (struct socketinf): Remove sin element.
(start_thread_socket): Initialize one SOCK_DGRAM socket per thread instead of
(apparently) expensive opening and closing of socket with each select call.
(socket_cleanup): Send a byte to the exitsock socket as a way to potentially
signal a waiting-for-socket thread to exit.
* tlsoffsets.h: Regenerate.