Merge #1923

1923: feat: I/O safety for 'sys/wait' r=asomers a=SteveLauC

#### What this PR does:
1. Adds I/O safety for `sys/wait`

----------

Actually, I am not sure about which type to use here:
```rust
pub enum Id<'fd> {
    /// Wait for the child referred to by the given PID file descriptor
    #[cfg(any(target_os = "android", target_os = "linux"))]
    PIDFd(RawFd),
    PIDFd(BorrowedFd<'fd>),
}
```

If we use `Fd: AsFd`

```rust
pub enum Id<'fd, Fd: AsFd> {
    /// Wait for the child referred to by the given PID file descriptor
    #[cfg(any(target_os = "android", target_os = "linux"))]
    PIDFd(RawFd),
    PIDFd(&'fd Fd),
}
```

 then the user has to specify that generic type when using this interface, which is kinda user-unfriendly...


------

The typical usage of this interface will be something like:

```rust
// Thought currently we don't have pidfd_open(2) in `Nix`
let fd_referring_to_a_process: OwnedFd  = pidfd_open().unwrap();
let status = waitid(Id::PIDFd(fd_referring_to_a_process), WaitPidFlag::XXXX).unwrap();
```

UPDATE: `pidfd_open(2)` will be added in #1859 or #1868 .


Co-authored-by: Steve Lau <stevelauc@outlook.com>

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