[C++0x] n2880 do not detach std::thread by default

[Paolo Carlini]

Jonathan Wakely wrote:
> n2880 changed the behaviour of std::thread so that it calls
> std::terminate() if the thread is joinable when destroyed or assigned
> to.
>
> I want to commit this to trunk and 4.4, but what's the best way to
> test it? (all current tests pass, because they always explicitly
> join() or detach() thread objects.)
>   
I would say, for now let's commit the update itself to both mainline and
branch.  If you think testing would be important, just add a one line
reminder right above the change.

Paolo.