[cpp-threads] pthreads cancellation

[Jason Merrill]

As I recall from those discussions, the solution I liked best for
synchronous cancellation was

1) disable cancellation during unwinding.
2) posix cancellation points throw a cancellation exception, which is a
   normal exception and can be caught and discarded.  If that happens,
   the destructor for the exception object requests cancellation again, and
   a new exception is thrown at the next cancellation point.

Awkwardly, Ulrich Drepper, the GNU libc maintainer, rejects any solution
that involves aborting the cancellation process once it has started.

Jason