Make DukeBots support co-op mode

Currently the bots ignore both Cooperative and Survival modes.

In order to support co-op, they need to stop ignoring monsters, be able to go through places like e1l1's roof, and stop sticking around their opening locations. Preferably they should start like the player without either weapons and items, then be able to pick them up.

There's already an addon called DukePlus that has great co-op bots, but they're stuck in the middle of many other unrelated features. Yet since it's open source, you can freely borrow from their code the parts you need.