Added I/O safety to sys::aio

[asomers]

What does this PR do

Adds I/O safety to the sys/aio module. Those methods now take AsFd arguments instead of RawFd. And struct Aiocb now has a lifetime argument.

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[SteveLauC]

Will review this tomorrow:)

[SteveLauC]

I found some minor issues that are not related to this PR, but the code of aio:

1. O_DSYNC seems to be available on FreeBSD

libc_enum! {
    /// Mode for `AioCb::fsync`.  Controls whether only data or both data and
    /// metadata are synced.
    #[repr(i32)]
    #[non_exhaustive]
    pub enum AioFsyncMode {
        /// do it like `fsync`
        O_SYNC,
        /// on supported operating systems only, do it like `fdatasync`
        #[cfg(any(apple_targets,
                  target_os = "linux",
                  netbsdlike))]
        O_DSYNC
    }
    impl TryFrom<i32>
}

#[cfg(any(
    freebsdlike,
    all(target_os = "linux", not(target_env = "uclibc")),
    apple_targets,
    target_os = "netbsd"
))]
feature! {
    #![feature = "aio"]
    pub mod aio;
}

O_DSYNC is available on OpenBSD, the aio module is not, though this won't harm anything.

2. There is an unnecessary type cast in the example of aio_suspend():

/// assert_eq!(aiocb.as_mut().aio_return().unwrap() as usize, WBUF.len());

aio_return() already returns an usize

[asomers]

Good find. I'll make a separate PR for those.