[Feature] Let AI use department channels directly (:h etc.)

[pali6]

Yesterday someone on Reddit mentioned that it sucks that AI can't use department channels directly. So I made a patch to add this as it seems like a reasonable QoL upgrade. The patch gives the AI a headset (a new "AI headset" that's really just renamed captain's headset). I guess this might be weird, maybe someone can yoink the headset from the AI or something, but it seemed like the least painful way. If we want to avoid problems like this it could probably be in a different variable as opposed to the ear slot just as AI radios are.

This is my first attempt at a patch here so please tell me if I screwed something up.

Here's the pull request.