BUGFIX : Repairing items, Buying merchants, GetEmptySlot()

Except for the final 2 (or 3) packets, buying merchants are fully
implemented. Repairing items now works (with region check incase the
user is botting) and GetEmptySlot was taking 2 arguments but never used
them.
Implemented CUser::FindSlotForItem() for efficient stack/free slot searching + cleaned up the buying merchant handler to be less confusing.
Tidied up a check in CUser::FindSlotForItem().
Fixing effect animations for single target healing spells
MOAR Buying Merchant.
Implemented temporary guard to prevent race conditions with AI (this won't be necessary soon).
Hackily reimplemented the rest (aside from the clan system) of the database request packets (need to do these properly at some point).
Hooked the clan system's database requests back up. They'll need to be done in more detail sometime, as there's no real reason to be re-parsing the packet -- better to just merge those handlers in.
Removed the _USER_DATA structure.
m_id is more appropriately now named m_strUserID (as per the column name), and is now an std::string. Also, GetName()'s used more viciously.
Removed the now-extremely-useless m_Accountid var, and added CUser::GetAccountName().
Moved some of the core Unit functionality back up into the Unit class (now that _USER_DATA's removed).
BUGFIX: Fixed NPC coordinates, so they display again (also, that check at the top should really not need to be there. Hmm.).
Removed CUser::RecvSkillDataLoad() & shifted it directly into CUser::ReqSkillDataLoad().
Removed CUser::RecvFriendProcess() & cleaned up the friend system database requests.
Removed CKngihtSManager::RecvCreateKnights() & shifted it into ReqCreateKnights().
Removed CKnightsManager::RecgDestroyKnights() and shifted it into ReqDestroyKnights().
Merged RecvKnightsList() with ReqKnightsList(). It also now loads the Ranking flag (as it apparently expected it to previously).
Replaced RecvRegisterClanSymbol() with ReqRegisterClanSymbol().
Forwarded ReqModifyKnightsMember() directly onto RecvModifyFame() (ugly, but if we're keeping UDP it's reusable).