Block pipe writes when the buffer grows too large

Created by: xTibor

It looks like the growth of the pipe buffers currently unbounded. This can cause problems when a pipe is written faster than the other end reads it.

For example for following test app produces an out of frames panic within seconds:

use std::process::{Command, Stdio};
use std::thread;

fn main() {
    let child = Command::new("cat")
        .args(&["zero:/"])
        .stdout(Stdio::piped())
        .spawn()
        .expect("failed to execute child");

    loop {
        thread::yield_now();
    }
}