--verbose: make it more efficient when not --verbose
In particular, don't require to call an externally defined function to find if in --verbose mode or not. Now it's just an extern bool. I'm confident the performance impact of not using -DNLOGDEBUG should be negligible now. This comes with a small incompatible API+ABI change, but that shouldn't matter as we've had a couple of those already since the last release.
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