Slack support

Not sure if anyone was planning on implementing this, but I'm on slack and figured why not? It probably needs some work. There is no modern rng-tools slack build available so I just pulled it from sourceforge, for example. Also, it asks the user to install some things if they aren't installed, which is reasonable for slackware imo because slackware has no native dependency checking and users are expected to deal with it themselves.  In any case, it compiles and runs on slackware for me using this. 
There's a `command_exists` function at the top of the file that can be used here for consistency and maintainability.