Fix features section handling in the fuzzer (#3980)
Fix features section handling in the fuzzer (#3980)

The features section is additive since #3960. For the fuzzer to know which features
are used, it therefore needs to also scan the features section. To do this,
run --print-features to get the total features used from both flags + the
features section.

A result of this is that we now have a list of enabled features instead of
"enable all, then disable". This is actually clearer I think, but it does require
inverting the logic in some places.
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