Prevent Pty::pts() from attaching via O_NOCTTY

[cpick]

If a process doesn't have a controlling terminal (for example if it is in a new session) then open(ptsname()) will automatically make the slave the controlling terminal of the process.

Later on, command.spawn(&pts)s call to ioctl_tiocsctty() within session_leader() within command.pre_exec() will fail as with EPERM since the parent process (and session) is alread using it as its controlling terminal.

Use O_NOCTTY to prevent this.

Tested on macOS and Linux.

[cpick]

To reproduce the issue compile:

fn main() {
    if std::env::args().len() > 1 {
        let _pid = rustix::process::setsid().expect("set sid");
    }

    let pty = pty_process::blocking::Pty::new().expect("new pty");
    let pts = pty.pts().expect("pty pts");

    let _child = pty_process::blocking::Command::new("true")
        .spawn(&pts)
        .expect("spawn command");
}

Then running it like so works:

sh -c '(target/debug/binary)'

But the following doesn't:

sh -c '(target/debug/binary broken)'
thread 'main' panicked at src/main.rs:11:10:
spawn command: Io(Os { code: 1, kind: PermissionDenied, message: "Operation not permitted" })
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Note: running it in a sub shell is needed so that it doesn't start out as a process group leader (which would cause setsid() to fail before ever getting to spawn()). Alternatively, the example could fork() and run in a child or call ioctl(1, TIOCNOTTY), but that would be more complicated example code.