Auto merge of #30490 - ipetkov:unix-spawn, r=alexcrichton

* If the requested descriptors to inherit are stdio descriptors there
  are situations where they will not be set correctly
* Example: parent's stdout --> child's stderr
           parent's stderr --> child's stdout
* Solution: if the requested descriptors for the child are stdio
  descriptors, `dup` them before overwriting the child's stdio

Example of a program which exhibits the bug:
```rust
// stdio.rs
use std::io::Write;
use std::io::{stdout, stderr};
use std::process::{Command, Stdio};
use std::os::unix::io::FromRawFd;

fn main() {
    stdout().write_all("parent stdout\n".as_bytes()).unwrap();
    stderr().write_all("parent stderr\n".as_bytes()).unwrap();

    Command::new("sh")
        .arg("-c")
        .arg("echo 'child stdout'; echo 'child stderr' 1>&2")
        .stdin(Stdio::inherit())
        .stdout(unsafe { FromRawFd::from_raw_fd(2) })
        .stderr(unsafe { FromRawFd::from_raw_fd(1) })
        .status()
        .unwrap_or_else(|e| { panic!("failed to execute process: {}", e) });
}
```

Before:
```
$ rustc --version
rustc 1.7.0-nightly (8ad12c3e2 2015-12-19)
$ rustc stdio.rs && ./stdio >out 2>err
$ cat out
parent stdout
$ cat err
parent stderr
child stdout
child stderr
```

After (expected):
```
$ rustc --version
rustc 1.7.0-dev (712eccee2 2015-12-19)
$ rustc stdio.rs && ./stdio >out 2>err
$ cat out
parent stdout
child stderr
$ cat err
parent stderr
child stdout
```

0 files changed, 0 insertions, 0 deletions