AI: make buzzers fly above the ground
AI: don't auto-request door cutting from non-bots
AI: a few tweaks

- enable new AI by default
- prefer disabled asw_marker over old-style markers
- medics will no longer heal the same target if there are two medics in
a squad
- medics go for the lowest-health target instead of the first target
- medics search for heal targets via squads instead of looking for
player marines
AI: director spawns buzzers higher in the air
AI: marines follow in a larger range

this should fix the last few marines not following on maps with sparse
marine hints.