SL(1): Cure your bad habit of mistyping

SL (Steam Locomotive) runs across your terminal when you type "sl" as you meant to type "ls". It's just a joke command, and not useful at all.

Update notes from eyJhb

I have merged the following

• Add SIGIN configuration and disco mode
• Add option for the number of cars

I have also added -v and updated the last updated and version number to 5.04.

Copyright 1993,1998,2014,2019 Toyoda Masashi (mtoyoda@acm.org)