Function nc::execve

pub fn execve(filename: &str, argv: &[&str], env: &[&str]) -> Result<(), Errno>

Execute a new program. 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::execve("/bin/ls", &args, &env);
    assert!(ret.is_ok());
}