Unfortunately, `std::string` elements are (bare) chars. As these
are the most likely type to be passed to these functions, make them use
char instead of unsigned char. This avoids some casts.
https://github.com/bitcoin/bitcoin/pull/15139/commits/332b3dd7c156daa08df347dc3359f0ee99ce4baa
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Depends on D6294
Concludes backport of Core PR15139