Add basics for async wait support

[MarkusBauer]

Currently, we can only block while we wait for a child process to terminate, which limits in many cases.
A non-blocking version of wait() - named try_wait() allows users to implement more features, for example timeouts or async/await support.

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For the reference: full async/await support could look like this snippet. This requires tokio as an additional dependency, thus I haven't added this code to the crate.

pub struct ChildWaiter<'a> {
    child: &'a mut Child,
    signal: tokio::signal::unix::Signal,
}

impl Future for ChildWaiter<'_> {
    type Output = io::Result<ExitStatus>;

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        // mostly borrowed from tokio's command/process handling
        loop {
            let registered_interest = self.signal.poll_recv(cx).is_pending();

            if let Some(status) = self.child.try_wait()? {
                return Poll::Ready(Ok(status));
            }

            if registered_interest {
                return Poll::Pending;
            } else {
                continue;
            }
        }
    }
}

/// Get a future that completes when the process terminates
pub fn wait_async(child: &mut Child) -> ChildWaiter {
    ChildWaiter {
        child,
        signal: signal(SignalKind::child()).expect("failed to register signal handler"),
    }
}

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Availability: If anyone wants to use this and other features before it's merged, check out my temporary fork. It includes #23 #27 #29 #33 #34 #35 and #36 with all conflicts resolved.
Add to your Cargo.toml: unshare = { git = "https://github.com/MarkusBauer/unshare.git" }