del'ing func.func_dict.  I took the opportunity to also clean up some
other nits with the code, namely core dumps when del'ing func_defaults
and KeyError instead of AttributeError when del'ing a non-existant
function attribute.

Specifically,

func_memberlist: Move func_dict and __dict__ into here instead of
special casing them in the setattro and getattro methods.  I don't
remember why I took them out of here before I first uploaded the PEP
232 patch. :/

func_getattro(): No need to special case __dict__/func_dict since
their now in the func_memberlist and PyMember_Get() should Do The
Right Thing (i.e. transforms NULL values into Py_None).

func_setattro(): Document the intended behavior of del'ing or setting
to None one of the special func_* attributes.  I.e.:

    func_code - can only be set to a code object.  It can't be del'd
    or set to None.

    func_defaults - can be del'd.  Can only be set to None or a tuple.

    func_dict - can be del'd.  Can only be set to None or a
    dictionary.

Fix core dumps and incorrect exceptions as described above.  Also, if
we're del'ing an arbitrary function attribute but func_dict is NULL,
don't create func_dict before discovering that we'll get an
AttributeError anyway.