SF bug #509805 tempfile.gettempdir not threadsafe

SF bug #509805 tempfile.gettempdir not threadsafe
This is an ancient race when multiple threads call gettempdir() (or
anything relying on it) for the first time.

Fixed x-platform via the Big Hammer of rearranging the code to serialize
the first calls.  Subsequent calls are as fast as before.

Note that the Python test suite can't provoke this bug:  it requires
setting up multiple threads making the very first calls into tempfile,
but the test suite uses tempfile several times before getting to
test_threadedtempfile.

Bugfix candidate.