Add EventManager.OnRegionLoginsStatusChange fired whenever logins are enabled or disabled at any point, not just during initial startup.

This replaces EventManager.OnLoginsEnabled which only fired when logins were first enabled
and was affected by a bug where it would never fire if the region started with logins disabled.

OpenSim/Framework/IScene.cs

@@ -66,6 +66,11 @@ namespace OpenSim.Framework
 
         IConfigSource Config { get; }
 
+        /// <summary>
+        /// Are logins enabled on this simulator?
+        /// </summary>
+        bool LoginsEnabled { get; set; }
+
         float TimeDilation { get; }
 
         bool AllowScriptCrossings { get; }