[cpp-threads] pthreads cancellation

[Doug Lea]

> 
> Given
> 
>         joiner<void> synch = thread(task);
> 
> Then either cancellation is expressed as a member of the joiner:
> 
>         synch.cancel();
> 
> Or it can be expressed as a non-member:
> 
>         cancel(synch);
> 


OK. Good (especially the former). And you'd also have isDone,
isCancelled methods? In which case I now see how the classes
are C++ analogs of Java Executors+Futures (so I like them :-)


-Doug