deny booting (DROP class improvements)

There was a long discussion on kea-users about being able to deny booting for some devices. In principle, it should be possible to add reservations with MAC address and class DROP for unwanted devices.

Here's what should be done:

1. Test if such configuration really discards the devices (there's code that checks DROP class at the end of Dhcpv4Srv::processPacket)
2. If it does, there should be a new section added to ARM that explains how to do this.
3. If it doesn't work, the code should be improved, so this is possible.

There's related request from a customer. See #1973 (closed) for details. RT #18741