[0.2] Small sync fixes and backports

[tgross35]

Just check out a few hunks of these files from main now that we aren't limited by the old MSRV..

Additionally, backport the following:

• https://github.com/rust-lang/libc/pull/3734
• https://github.com/rust-lang/libc/pull/3746
• https://github.com/rust-lang/libc/pull/3617
• https://github.com/rust-lang/libc/pull/3583
• https://github.com/rust-lang/libc/pull/3213
• https://github.com/rust-lang/libc/pull/4087
• https://github.com/rust-lang/libc/pull/3647

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