Retire snprintf replacement or replace with c99-snprintf

[eagle]

The snprintf replacement from rra-c-util requires a lot of ongoing maintenance to compile cleanly with new warning options, and it's unclear that a replacement snprintf still serves any real portability objective. Systems without a working snprintf are now fairly old, and I'm considering dropping it from rra-c-util to reduce maintenance work.

Consider dropping this from INN and just requiring a working snprintf for INN to build, or if we want to keep it, replace it with c99-snprintf from ​https://github.com/weiss/c99-snprintf, which is being (somewhat) actively maintained. That version probably doesn't compile cleanly with all the warnings we use, but it's somewhat cleaner code.