Function nc::execveat

pub fn execveat(
    fd: i32, 
    filename: &str, 
    argv: &[&str], 
    env: &[&str], 
    flags: i32
) -> Result<(), Errno>

Execute a new program relative to a directory file descriptor. TODO(Shaohua): type of argv and env will be changed. And return value might be changed too.

let pid = nc::fork();
assert!(pid.is_ok());
let pid = pid.unwrap();
assert!(pid >= 0);
if pid == 0 {
    // child process
    let args = [""];
    let env = [""];
    let ret = nc::execveat(nc::AT_FDCWD, "/bin/ls", &args, &env, 0);
    assert!(ret.is_ok());
}