Variety of small INC/DECREF patches that fix reported memory leaks

Variety of small INC/DECREF patches that fix reported memory leaks
with free variables.  Thanks to Martin v. Loewis for finding two of
the problems.  This fixes SF buf 405583.

There is also a C API change: PyFrame_New() is reverting to its
pre-2.1 signature.  The change introduced by nested scopes was a
mistake.  XXX Is this okay between beta releases?

cell_clear(), the GC helper, must decref its reference to break
cycles.

frame_dealloc() must dealloc all cell vars and free vars in addition
to locals.

eval_code2() setup code must INCREF cells it copies out of the
closure.

The STORE_DEREF opcode implementation must DECREF the object it passes
to PyCell_Set().