PyEval_SaveThread() and PyEval_RestoreThread() now return/take a

PyEval_SaveThread() and PyEval_RestoreThread() now return/take a
PyThreadState pointer instead of a (frame) PyObject pointer.  This
makes much more sense.  It is backward incompatible, but that's no
problem, because (a) the heaviest users are the Py_{BEGIN,END}_
ALLOW_THREADS macros here, which have been fixed too; (b) there are
very few direct users; (c) those who use it are there will probably
appreciate the change.

Also, added new functions PyEval_AcquireThread() and
PyEval_ReleaseThread() which allows the threads created by the thread
module as well threads created by others (!) to set/reset the current
thread, and at the same time acquire/release the interpreter lock.

Much saner.