Print debug warning on stderr, not stdout.

[athas]

This prevents things like list-bin from being broken.

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[ulysses4ever]

A more correct version of it has already been accepted and currently awaits a merge at #8396, so I suggest closing this. But thank you for taking the effort to fix it anyway!

[Mikolaj]

@athas: if you could make a lightning review of #8396, that's always valuable.