Entities/Unit: Finally fix no-path evasion.

- When a creature cannot find a path to its victim, it begins evading all attacks and regenerating health.
- If this persists for 5 seconds, it evades back to spawn position with new EvadeReason value EVADE_REASON_NO_PATH.
- Also some SmartAI cleanup (why oh why does it have so much duplicated code) and getting rid of #defines in favor of type-checked compile-time constants.

src/server/game/AI/SmartScripts/SmartAI.cpp

@@ -416,12 +416,7 @@ void SmartAI::EnterEvadeMode(EvadeReason /*why*/)
     me->RemoveAllAurasExceptType(SPELL_AURA_CONTROL_VEHICLE, SPELL_AURA_CLONE_CASTER);
 
     me->AddUnitState(UNIT_STATE_EVADE);
-    me->DeleteThreatList();
-    me->CombatStop(true);
-    me->LoadCreaturesAddon();
-    me->SetLootRecipient(NULL);
-    me->ResetPlayerDamageReq();
-    me->SetLastDamagedTime(0);
+    _EnterEvadeMode();
 
     GetScript()->ProcessEventsFor(SMART_EVENT_EVADE);//must be after aura clear so we can cast spells from db