Try to repair what may be the last new test failure on the

Try to repair what may be the last new test failure on the
"x86 OpenBSD trunk" buildbot due to changing Python so that
Python-exposed addresses are always non-negative.

test_int_pointer_arg():  This line failed now whenever the
box happened to assign an address to `ci` "with the sign
bit set":

    self.failUnlessEqual(addressof(ci), func(byref(ci)))

The problem is that the ctypes addressof() inherited "all
addresses are non-negative now" from changes to
PyLong_FromVoidPtr(), but byref() did not inherit that
change and can still return a negative int.

I don't know whether, or what, the ctypes implementation wants
to do about that (possibly nothing), but in the meantime
the test fails frequently.

So, introduced a Python positive_address() function in
the test module, that takes a purported machine address and,
if negative, converts it to a non-negative value "with the
same bits".  This should leave the test passing under all
versions of Python.

Belated thanks to Armin Rigo for teaching me the sick trick ;-)
for determining the # of bits in a machine pointer via abuse
of the struct module.