Replace occurences of uninitialized with MaybeUninit

mem::uninitialized is deprecated, so move over the not-UB MaybeUninit.

I see that the &mut references to MaybeUninit stuff are generally replaced by raw pointers, however I am also aware that taking references to MaybeUninit is currently unstable (tracking issue: https://github.com/rust-lang/rust/issues/63568). I am not sure how this fits into relibc's policy of preferring references over pointers "where applicable"?

The functions using MaybeUninit are generally calling "non-owning" variants of the C calls. Either we must use pointers, or we duplicate the checks and call the Sys directly. I think calling the functions is more maintainable in the long term. That said I'd have no problem making only references is this is deemed preferable.

added 25 commits

• 7a460e0e...7d6288ab - 24 commits from branch master
• 8ee5c0d8 - Replace occurences of uninitialized with MaybeUninit

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mentioned in commit c6e3cb8e