free thread related memory created by pthread_create()

[Florian Weimer]

* Joël Krähemann:

> The very last entry of following shows the issue with pthread_create():
>
> http://pastebin.com/aQfAQV2q

Usually, this is due to a race condition at process termination.
valgrind may or may not report the resources of a thread still running
at process termination as leaked.

You can avoid the valgrind warning by delaying exit until all other
threads have terminated.

It is not a memory leak in the classic because this happen only during
process termination, just before the kernel is about to free all
process memory anyway.