Make default instruction text for “not applicable” configurable

[nstanger]

Apparently the current vogue is to say “Nil” in the instructions on the front page. Rather than have to change things every time they change their mind, let’s just put this text in a re-definable macro that defaults to “N/A” but can be easily set to whatever.

[nstanger]

I should read my own documentation. You can already (and have always been able to) override the default by explicitly calling the likes of \copiesof with an argument. Duh.