Add const to several API functions that take char *.

In C++, it's an error to pass a string literal to a char* function
without a const_cast().  Rather than require every C++ extension
module to put a cast around string literals, fix the API to state the
const-ness.

I focused on parts of the API where people usually pass literals:
PyArg_ParseTuple() and friends, Py_BuildValue(), PyMethodDef, the type
slots, etc.  Predictably, there were a large set of functions that
needed to be fixed as a result of these changes.  The most pervasive
change was to make the keyword args list passed to
PyArg_ParseTupleAndKewords() to be a const char *kwlist[].

One cast was required as a result of the changes:  A type object
mallocs the memory for its tp_doc slot and later frees it.
PyTypeObject says that tp_doc is const char *; but if the type was
created by type_new(), we know it is safe to cast to char *.

Include/methodobject.h

@@ -35,15 +35,15 @@ PyAPI_FUNC(int) PyCFunction_GetFlags(PyObject *);
 PyAPI_FUNC(PyObject *) PyCFunction_Call(PyObject *, PyObject *, PyObject *);
 
 struct PyMethodDef {
-    char	*ml_name;	/* The name of the built-in function/method */
+    const char	*ml_name;	/* The name of the built-in function/method */
     PyCFunction  ml_meth;	/* The C function that implements it */
     int		 ml_flags;	/* Combination of METH_xxx flags, which mostly
 				   describe the args expected by the C func */
-    char	*ml_doc;	/* The __doc__ attribute, or NULL */
+    const char	*ml_doc;	/* The __doc__ attribute, or NULL */
 };
 typedef struct PyMethodDef PyMethodDef;
 
-PyAPI_FUNC(PyObject *) Py_FindMethod(PyMethodDef[], PyObject *, char *);
+PyAPI_FUNC(PyObject *) Py_FindMethod(PyMethodDef[], PyObject *, const char *);
 
 #define PyCFunction_New(ML, SELF) PyCFunction_NewEx((ML), (SELF), NULL)
 PyAPI_FUNC(PyObject *) PyCFunction_NewEx(PyMethodDef *, PyObject *, 
@@ -76,7 +76,7 @@ typedef struct PyMethodChain {
 } PyMethodChain;
 
 PyAPI_FUNC(PyObject *) Py_FindMethodInChain(PyMethodChain *, PyObject *,
-                                                  char *);
+                                            const char *);
 
 typedef struct {
     PyObject_HEAD