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...

Objects/unicodeobject.c

@@ -2384,6 +2384,7 @@ PyObject *_PyUnicode_DecodeUnicodeInternal(const char *s,
     Py_UNICODE unimax = PyUnicode_GetMax();
 #endif
 
+    /* XXX overflow detection missing */
     v = _PyUnicode_New((size+Py_UNICODE_SIZE-1)/ Py_UNICODE_SIZE);
     if (v == NULL)
 	goto onError;
@@ -3170,6 +3171,7 @@ PyObject *PyUnicode_DecodeCharmap(const char *s,
 			Py_ssize_t needed = (targetsize - extrachars) + \
 				     (targetsize << 2);
 			extrachars += needed;
+			/* XXX overflow detection missing */
 			if (_PyUnicode_Resize(&v,
 					     PyUnicode_GET_SIZE(v) + needed) < 0) {
 			    Py_DECREF(x);
@@ -7762,10 +7764,11 @@ PyObject *PyUnicode_Format(PyObject *format,
 	    default:
 		PyErr_Format(PyExc_ValueError,
 			     "unsupported format character '%c' (0x%x) "
-			     "at index %i",
+			     "at index %zd",
 			     (31<=c && c<=126) ? (char)c : '?',
                              (int)c,
-			     (int)(fmt -1 - PyUnicode_AS_UNICODE(uformat)));
+			     (Py_ssize_t)(fmt - 1 -
+					  PyUnicode_AS_UNICODE(uformat)));
 		goto onError;
 	    }
 	    if (sign) {