added a random rulenum to ipfw created rule

If more than one rule is created during the banTime what happens is that when the unban time occurs all ipfw rules are removed even if some of them haven't completed the assigned banTime. As ipfw assigns the same rulenum to every rule when none is explicitly given. The actionunban thereby becomes overly aggressive.

I've had this in place on my OSX 10.6.8 server for many years. Please let me know if you have any questions.

I hope I'm doing this correctly. This is my first pull request.