CO_FUTURE_DIVISION flag.  Redid this to use Jeremy's PyCF_MASK #define
instead, so we dont have to remember to fiddle individual feature names
here again.

pythonrun.h:  Also #define a PyCF_MASK_OBSOLETE mask.  This isn't used
yet, but will be as part of the PEP 264 implementation (compile() mustn't
raise an error just because old code uses a flag name that's become
obsolete; a warning may be appropriate, but not an error; so compile() has
to know about obsolete flags too, but nobody is going to remember to
update compile() with individual obsolete flag names across releases either
-- i.e., this is the flip side of PyEval_MergeCompilerFlags's oversight).