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...

Lib/test/test_builtin.py

@@ -156,6 +156,11 @@ class BuiltinTest(unittest.TestCase):
         S = [10, 20, 30]
         self.assertEqual(any(x > 42 for x in S), False)
 
+    def test_neg(self):
+        x = -sys.maxint-1
+        self.assert_(isinstance(x, int))
+        self.assertEqual(-x, sys.maxint+1)
+
     def test_apply(self):
         def f0(*args):
             self.assertEqual(args, ())
@@ -702,9 +707,11 @@ class BuiltinTest(unittest.TestCase):
                         pass
 
         s = repr(-1-sys.maxint)
-        self.assertEqual(int(s)+1, -sys.maxint)
+        x = int(s)
+        self.assertEqual(x+1, -sys.maxint)
+        self.assert_(isinstance(x, int))
         # should return long
-        int(s[1:])
+        self.assertEqual(int(s[1:]), sys.maxint+1)
 
         # should return long
         x = int(1e100)