ftpusers(5) example: use IP & DNS examples

Use .example TLD, per IETF RFC 2606.
Use 192.0.2.0/24, per IETF RFC 5737.
Use 127.0.0.1/24 for localguest, not network that ftp.NetBSD.org used to be in.

netbsd.org->NetBSD.org

improve examples:
- put local anon users into a separate class `localguest'
- use `localguest' to demonstrate the template features
- provide a better description of how the conversion commands work.

sample ftpusers with lots of examples on the syntax