Close SF bug 110826: a complaint about the way Python #define'd NULL.

It's hard to sort out what the bug was, exactly.  So, Big Hammer:

1. Python shouldn't be in the business of #define'ing NULL, period.
2. Users of the Python C API shouldn't be in the business of not including
   Python.h, period.

Hence:

1. Removed all #define's of NULL in Python source code (pyport.h and
   object.h).
2. Since we're *relying* on stdio.h defining NULL, put an #error in
   Python.h after its #include of stdio.h if NULL isn't defined then.

Include/Python.h

@@ -36,6 +36,10 @@
 #endif
 
 #include <stdio.h>
+#ifndef NULL
+#   error "Python.h requires that stdio.h define NULL."
+#endif
+
 #include <string.h>
 #include <errno.h>
 #ifdef HAVE_STDLIB_H