Implemented EventFlag system to control quest availability:
- Added IsEventFlagSet() to check if EventFlag exists in questmanager
- Added EventFlag checks in ExecuteQuestScript() for per-quest control
- Added EventFlag checks in HandleEvent() for NPC-based quest control
- Added EventFlag checks in OnChat() for chat-based quest control
Usage:
- /event quest_<questname>_enabled 0/1 - Enable/disable specific quest
- /event quest_npc_<vnum>_enabled 0/1 - Enable/disable all NPC quests
- /event quests_global_enabled 0/1 - Enable/disable all quests globally
