Commands implementation

In the previous code, we only handled one command; any other command will receive an unknown command answer from the server. Also, our Auth implementation says it's not implemented. So, to sum this up, we handle one command that answers that it's not implemented. Crazy, right? For the Auth command, we'll look at this later.

Now let's implement some commands for real. Let's start with a simple one: Syst. This is supposed to return which system this FTP server is running on. For some reason, we won't answer that, and we'll just send back an answer-nothing usable.