coda-ascbin-cursor.c: Localize variables and add const when possible

[schwehr]

Demonstration of #48. I understand that the project will reject this patch.

[svniemeijer]

I don't see how this adds anything. Static analysers should be able to handle both cases similarly nowadays. In that case it just becomes a matter of style (and what you find more readable).

It is indeed rejected as already clarified in #48.