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

[Jonathan Wakely]

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.)

Index: include/std/thread
===================================================================
--- include/std/thread  (revision 148467)
+++ include/std/thread  (working copy)
@@ -135,7 +135,7 @@ namespace std
     ~thread()
     {
       if (joinable())
-       detach();
+       std::terminate();
     }

     thread& operator=(const thread&) = delete;
@@ -143,7 +143,7 @@ namespace std
     thread& operator=(thread&& __t)
     {
       if (joinable())
-       detach();
+       std::terminate();
       swap(__t);
       return *this;
     }