Forward-port of r52136: 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/sysmodule.c

@@ -1225,7 +1225,7 @@ makepathobject(char *path, int delim)
 		p = strchr(path, delim);
 		if (p == NULL)
 			p = strchr(path, '\0'); /* End of string */
-		w = PyString_FromStringAndSize(path, (int) (p - path));
+		w = PyString_FromStringAndSize(path, (Py_ssize_t) (p - path));
 		if (w == NULL) {
 			Py_DECREF(v);
 			return NULL;