Try and fix unbind_to when localness of binding has changed

Try and fix unbind_to when localness of binding has changed
* eval.c (unbind_to): Don't unbind a local binding into the global
binding when the local binding disappeared.  Inversely, don't unbind
a global binding into a newly created local binding.
* data.c (set_internal): Make its `buf' arg into a `where' arg so we
can specify the frame to use, when applicable.  Adjust callers.