Remove most linux-raw-sys types from the public API

[sunfishcode]

Add locally defined replacements to avoid depending on bindgen's __IncompleteArrayField and __BindgenUnionField in linux-raw-sys, use crate::ffi instead of crate::backend::c where applicable, remove WaitIdStatus::as_raw since it only exists to expose a linux-raw-sys type, and add locally defined replacements for Winsize and Sysinfo.

In all, these changes eliminate most of the linux-raw-sys types from the public API.