Implement confstr (unistd.h)
For Fish. Musl's source amusingly calls this function entirely useless.
I also switched an #[unsafe(no_mangle)] from my last patch back to a #[no_mangle] because we need to bump cbindgen. #[unsafe(no_mangle)] is required for Rust 2024. I'm not sure why it builds in CI and locally though.
The implementation for confstr is straightforward. Most of the constants return 0 on both musl and glibc. The only constant that doesn't is required for Fish.