Refactor paradrops

* The paradrops thread now checks to see how many helipads (which are
placed invisibly by the paradrop script) are present near the chosen
player, instead of trying to keep track of active paradrop units.
* Remove mission-specific logic from vendor/lv/reinforcementChopper.sqf.
* Add nil checks to BLOL_fnc_players_closestDistanceTo.
* Add BLOL_fnc_ambiance_paradrops.
* Add BLOL_fnc_ambiance_paradrop.