Overhauled setAISkill

- touches individual skill sets instead of total skill value (values might need tweaking)
- friendly AI for coop are defined in a single array in init.sqf
- removed code redundancy:
main file is now f_setAISkill and f_setAISkillAD or f_setAISkillCOOP are included depending on which parameters do exist (are !nil)
- new function: f_fnc_setAISkill
sets individual skill values for a unit.
adds variable "f_setAISkill" to a unit, flagged to true

- f_setAISkill can differentiate between units that it already modified thus can be run several times in a mission to affect newly spawned AI (alternative would be a loop checking so-and-so often)
- f_fnc_setAISkill can be called for individual units to override skill levels set by f_setAISkill