* Add single arg version Unit::AddThreat for just adding to threat list.
* Req. provide schollmask and crit flag for any real threat value for proper threats mod apply.
* Send crit flag in DealDamage as MELEE_HIT_CRIT for spell damage for later send to threat call.
* For not affected by modifiers threat values use SPELL_SCHOOL_MASK_NONE.
* Implement aura SPELL_AURA_MOD_CRITICAL_THREAT (used only in itemset 529 effect).
Also send pointer to AI constructors ans mark constructors as explicit.
This changes allow move now some generic often used AI code to CreatureAI helper functions.
