realpath and relpath implementation

Realpath (#315) provides set of features similar to the plain Debian implementation. GNU options aren't supported yet.
Relpath (#316) options are similar to the old one FreeBSD relpath package, since no coreutils implementation currently providing this command.
Can you change this to:
```rust
match std::io::fs::readlink(&result) {
    Ok(x) => {
        result.pop();
        result.push(x);
    }
    _ => {
        if !quiet {
            show_error!("Invalid path: {}", path);
        }
        false
    }
}
```