Implement thrd_detach.
Call CloseHandle in thrd_join on Windows.

https://github.com/nemequ/tinycthread/issues/6
Switch from _beginthreadex to CreateThread on Windows.

According to the MSDN docs, "This API cannot be used in applications
that execute in the Windows Runtime or are compiled by using the
clr:pure switch."  Switching to CreateThread should provide better
portability and, AFAICT, we don't really lose anything.
Add unit test for thrd_exit.
Add documentation for thrd_detach.

https://github.com/nemequ/tinycthread/issues/7
Fix use-after-free in TSS cleanup on Windows.

https://github.com/nemequ/tinycthread/issues/8
Switch unit tests to C89 mode and fix errors it caused.

https://github.com/nemequ/tinycthread/issues/9