--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.