This allows unique identification across saves. The front-end needs to
be able to differentiate the first carrier in game A and the first
carrier in game B, but because carriers (and other non-airfield CPs) are
assigned IDs sequentially, collisions were to be expected. The front-end
can't tell the difference between a reloaded game and a new turn, so we
need to ensure different IDs across games.
This is a handy cleanup anyway, since callers constructing CPs no longer
need to manually track the CP ID counter.
Fixes https://github.com/dcs-liberation/dcs_liberation/issues/2078.
