Moved the code for _PyThread_CurrentFrames() up, so it's no longer
in a huge "#ifdef WITH_THREAD" block (I didn't realize it /was/ in
one).

Changed test_sys's test_current_frames() so it passes with or without
thread supported compiled in.

Note that test_sys fails when Python is compiled without threads,
but for an unrelated reason (the old test_exit() fails with an
indirect ImportError on the `thread` module).  There are also
other unrelated compilation failures without threads, in extension
modules (like ctypes); at least the core compiles again.

Do we really support --without-threads?  If so, there are several
problems remaining.