* 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).
