Black-list protection rule like fail2ban

[cameyo]

In RdpMon.cs there is a commented section "BlackList protection rule: TBD".
It implements automatic black-listing of IP addresses after a certain failure threshold. It basically works, but requires some polishing before it can be put to use:
#1- UI for configuring, enabling and disabling the protection rule.
#2- Some forgetting range after which IPs are OK again. If a legitimate user has put the wrong password more than 10 times over the course of a long period of time (i.e. throughout an entire month) or... because they were really confused, we still want to give them another chance after a few hours for example. This has to be included in the configuration UI mentioned in #1 of course.