POSIX patch

[Vladimir Nesic]

I have found an error preventing pthread_cleanup_pop() and  
_POSIX_Thread_cancel_run() from running correctly.

In both functions I have changed line:

handler = (POSIX_Cancel_Handler_control *) _Chain_Tail( handler_stack );

to:

handler = (POSIX_Cancel_Handler_control *) _Chain_Tail( handler_stack 
)->previous;

and now everything seams to work OK.

Vlada