1.0.24.28: make unbind in the runtime zero the value of the binding

1.0.24.28: make unbind in the runtime zero the value of the binding

... to parallel what the Lisp side does. As far as I see the rationale
for doing that (in doc/internals/specials.texinfo) does not apply here
since signals are blocked when the dynbind functions are called. That
said, instead of adding documentation let's fix it.