Move randof() to become a real function

[jimklimov]

While the pieces of modular randof implementation got nicely spread around, the routine could benefit from becoming a real function (or a set of those) wrapping the same coding patterns and platform-dependent quirks. In particular, being a function would allow to better address run-time discrepancies between huge num range and a small RAND_MAX spectrum, as well as to compile changes once rather than require rebuild of CZMQ and whoever uses the header.

This issue is a sort of request for info: was there any special purpose to making it a macro rather than function in the first place?

[stale]

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