Forward-port of r52136,52138: a review of overflow-detecting code.

* unified the way intobject, longobject and mystrtoul handle
  values around -sys.maxint-1.

* in general, trying to entierely avoid overflows in any computation
  involving signed ints or longs is extremely involved.  Fixed a few
  simple cases where a compiler might be too clever (but that's all
  guesswork).

* more overflow checks against bad data in marshal.c.

* 2.5 specific: fixed a number of places that were still confusing int
  and Py_ssize_t.  Some of them could potentially have caused
  "real-world" breakage.

* list.pop(x): fixing overflow issues on x was messy.  I just reverted
  to PyArg_ParseTuple("n"), which does the right thing.  (An obscure
  test was trying to give a Decimal to list.pop()... doesn't make
  sense any more IMHO)

* trying to write a few tests...

Python/errors.c

@@ -551,7 +551,8 @@ PyErr_NewException(char *name, PyObject *base, PyObject *dict)
 			goto failure;
 	}
 	if (PyDict_GetItemString(dict, "__module__") == NULL) {
-		modulename = PyString_FromStringAndSize(name, (int)(dot-name));
+		modulename = PyString_FromStringAndSize(name,
+						     (Py_ssize_t)(dot-name));
 		if (modulename == NULL)
 			goto failure;
 		if (PyDict_SetItemString(dict, "__module__", modulename) != 0)