pthread_cancel and EH: let's try this again

[Alexander Terekhov]

Peter Dimov wrote:
[...]
> int pthread_cancelenabled(void);
> 
> RETURN VALUE
> 
> Nonzero if the current thread's cancelability state is PTHREAD_CANCEL_ENABLE
> and the call is not made from within a cleanup handler or a TSD destructor,
                                ^^^^^^^^^^^^^^^^^^^^^^^^
> zero otherwise.
> 
> could have been much simpler.

With cancel request pending, mandatory cancel points are required to 
throw within a cleanup handler invoked by pthread_cleanup_pop() ["(if 
/execute/ is non-zero)"] when cancel state is PTHREAD_CANCEL_ENABLE.

regards,
alexander.