Some cleanups for https://github.com/fontforge/fontforge/pull/1843 and other

This has some other cleanups to bring down warning message sizes too
Personally, I'd write a zuuid_extra.h header file and call it ZUUID_STR_LEN. This constant should be defined by `zuuid.h`. Worrying about using another library in future (why would we? CZMQ won't be dropping its implementation, and it doesn't lack anything we need) and/or guaranteeing that FF will stick to the same length for UUIDs (I don't see how you can guarantee that without picking a standard UUID format, and since zuuid.h doesn't guarantee that its strings are of a particular format, then we'd have to go back to ossp-uuid or some other implementation with guaranteed formats) is all over-engineering what is (and should remain) an isolated use of a feature: at present, used only once, so abstracting it within Fontforge doesn't gain anything.