sched::clone is unsound and should be unsafe to call

[djkoloski]

Reproduced on Debian 5.19.11:

use nix::sched::{clone, CloneFlags};

fn main() {
    let mut bytes = [0u8; 1024];
    let cb = Box::new(|| 0isize) as Box<dyn FnMut() -> isize>;
    let _ = clone(cb, &mut bytes[1024..], CloneFlags::CLONE_VM, None);
    for byte in bytes {
        assert_eq!(byte, 0);
    }
}

Output:

thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `48`,
 right: `0`', src/main.rs:8:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
*** stack smashing detected ***: terminated
Segmentation fault

This function should be unsafe to call. Related: #360