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.
I checked that way back when the change was made. It's not dangerous. The reason had something to do with the Starport space frigate getting permamently stuck on the Starport when unloading carryalls, because they had no valid cells to exit to (since they are aircraft they never needed that before, so it was `Invalid`).