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

This has some other cleanups to bring down warning message sizes too
I noticed that before, https://github.com/fontforge/fontforge/pull/1843#discussion_r19378264
and the code above mentions that it is based on the czmq constant.

its a hard choice, because czmq is optional then gnetwork would need to include czmq header optionally (after config) and its then another constant that might trip up optional code or declarations. Since it doesn't change in size I've noted in the comment that it is actually equal to the ZUUID_LEN from czmq. But redeclaring it there means that some other declarations might be made in a non czmq enclosed block.