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

[Benjamin Kosnik]

> n2880 changed the behaviour of std::thread so that it calls
> std::terminate() if the thread is joinable when destroyed or assigned
> to.

You can test that terminate has been called by setting terminate to a
test handler via set_terminate. The test handler can just set
some switch or variable, and the testsuite can ping that. It's similar
to testing that new/delete are called.

-benjamin