* cygthread.h (cygthread::terminate): Declare new function.

* cygthread.h (cygthread::terminate): Declare new function.
(cygthread::initialized): Change to 'int'.
* cygthread.cc (cygthread::stub): Exit thread if initialized < 0.
(cygthread::new): Ditto.
(cygthread::runner): Ditto.  Set initialized using xor to preserve sign.
(cygthread::terminate): New function.
* dcrt0.cc (do_exit): Call cygthread::terminate.