Better Airdrop Production

This change allows the Production class and its children to take in multiple actors to be produced.
This is needed for #7055 so that the units produced can exit in an orderly fashion.
Not really: `RallyPoint` is a production-specific trait, so all this does is couple the two otherwise-unrelated groups of code together.  This could be avoided by passing in a target cell instead of the rally point trait.