Consistently handle child prefix/index as unsigned char, funsigned-char

[evelikov]

As a follow-up to https://github.com/kmod-project/kmod/pull/247 it addresses all the s/int/unsigned char/ instances that I could find.

It also forces char to unsigned char, since that's the majority(?) of our use case.

NOTE: the PR is completely untested, hence marked as draft

Once the linked PR has landed and we have agreement if -funsigned-char is something we'd really want, I will grab a few old and new kernels and do some testing.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 60.00000% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
libkmod/libkmod-index.c	45.45%	6 Missing :warning:

Files with missing lines	Coverage Δ
tools/depmod.c	57.16% <100.00%> (ø)
libkmod/libkmod-index.c	50.32% <45.45%> (ø)

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[lucasdemarchi]

Applied, thanks.