Add tnftp_savefile exploit

Also adds ```URI{HOST,PORT}``` and ```{,v}print_good``` to ```HttpServer```.

**Verification steps:**

- [ ] Start the server (I like using an innocuous ```URIPATH```)
- [ ] Connect to it with ```ftp``` on OS X or FreeBSD or NetBSD or DragonFly BSD or...?
- [ ] See that ```CMD``` gets executed on the client
Had to do it to work around my tunnels. Could extract host and port from the ```Host``` header, but I think short-circuiting that logic was better. Some clients might not send ```Host``` (HTTP/1.0), etc.