Track the creating plugin for convars (#1537)

Similar to the recent work for commands, track and expose the creating
plugin for convars. The first plugin to register a given cvar becomes
the owner until that plugin is unloaded. If a plugin attempts to
register a convar that was already registered and the originally
registering plugin has been unloaded, that plugin becomes the owner.
This isn't quite as nice as the way commands shift "ownership" as
plugins are unloaded, but we don't have a sane data structure currently
to implement that, and it seemed like a lot of unnecessary work as there
shouldn't really be multiple plugins with conflicting cvars.

Closes #1492

core/ConVarManager.h

@@ -62,6 +62,7 @@ struct ConVarInfo
 	bool sourceMod;						/**< Determines whether or not convar was created by a SourceMod plugin */
 	IChangeableForward *pChangeForward;	/**< Forward associated with convar */
 	ConVar *pVar;						/**< The actual convar */
+	IPlugin *pPlugin; 					/**< Originally owning plugin */
 	List<IConVarChangeListener *> changeListeners;
 
 	struct ConVarPolicy
@@ -154,7 +155,7 @@ public:
 
 	bool IsQueryingSupported();
 
-	HandleError ReadConVarHandle(Handle_t hndl, ConVar **pVar);
+	HandleError ReadConVarHandle(Handle_t hndl, ConVar **pVar, IPlugin **ppPlugin = nullptr);
 
 	// Called via game hooks.
 	void OnConVarChanged(ConVar *pConVar, const char *oldValue, float flOldValue);
@@ -171,7 +172,7 @@ private:
 	/**
 	 * Adds a convar to a plugin's list.
 	 */
-	static void AddConVarToPluginList(IPluginContext *pContext, const ConVar *pConVar);
+	static void AddConVarToPluginList(IPlugin *plugin, const ConVar *pConVar);
 private:
 	HandleType_t m_ConVarType;
 	List<ConVarInfo *> m_ConVars;