Add new function "local_scope" to label subsystem to
return the previous non-local label for a given local label,
and invoke this funcion in eval.c to display the fully
qualified name in the "not defined" error message.
Proper use of bool and enum makes code easier to debug. Do more of
it. In particular, we really should stomp out any residual uses of
magic constants that aren't enums or, in some cases, even #defines.