[7668] Cleanup In CreatureAI function descriptions and AttackStart/AttackedBy use.

* Use AI::AttackStart calls only in case explicit request creature attack from core or AI code "attack it if can".
  Like taunt, pet handler attack command.
* Use AI::AttackedBy for reaction at hostile action "do something at hostile action"
  Like non-dot damage, swing, negative spell landing, or fade fear/etc.
  And provided by default call AttackStart if no current target.

This fix some problems, like:
* Civilian will react propertly at attack by another creature (not pet or player).
* Will not cases (at least triggred by core) when attack target start run to attacker before any real hostile action apply.

src/game/CreatureAI.cpp

@@ -17,7 +17,14 @@
  */
 
 #include "CreatureAI.h"
+#include "Creature.h"
 
 CreatureAI::~CreatureAI()
 {
 }
+
+void CreatureAI::AttackedBy( Unit* attacker )
+{
+    if(!m_creature->getVictim())
+        AttackStart(attacker);
+}