* thread.cc (pthread::create): Handle stackaddr as upper bound address.

	* thread.cc (pthread::create): Handle stackaddr as upper bound address.
	Add comment.
	(pthread_attr_setstack): Store upper bound address in stackaddr.
	Explain why.
	(pthread_attr_getstack): Handle stackaddr as upper bound address.
	Add comment.
	(pthread_attr_setstackaddr): Add comment.
	(pthread_attr_getstackaddr): Add comment.
	(pthread_attr_getstacksize): Return default stacksize if stacksize has
	not been set by the application, just as on Linux.  Add comment.
	(pthread_getattr_np): Store upper bound address in stackaddr.  Explain
	why.
	* include/pthread.h: Remove outdated comment.
	(pthread_attr_getstackaddr): Mark as deprecated, as on Linux.
	(pthread_attr_setstackaddr): Ditto.