pyport.h: typedef a new Py_intptr_t type.
DELICATE ASSUMPTION: That HAVE_UINTPTR_T implies intptr_t is
available as well as uintptr_t. If that turns out not to be
true, things must get uglier (C99 wants both, so I think it's
an assumption we're *likely* to get away with).
thread_nt.h, PyThread_start_new_thread: MS _beginthread is documented
as returning unsigned long; no idea why uintptr_t was being used.
Others: Always use Py_[u]intptr_t, never [u]intptr_t directly.