For scripts in attachments, don't save .state files apart from the initial one as these are ignored since .state is saved in the attachment's asset.

This eliminates pointless work and exceptions when an appdomain is unloaded whilst an attachment script state is persisted.
Adds test for this case.
Relates to http://opensimulator.org/mantis/view.php?id=7407

OpenSim/Region/ScriptEngine/Interfaces/IScriptInstance.cs

@@ -97,6 +97,12 @@ namespace OpenSim.Region.ScriptEngine.Interfaces
         /// </summary>
         string State { get; set; }
 
+        /// <summary>
+        /// If true then the engine is responsible for persisted state.  If false then some other component may 
+        /// persist state (e.g. attachments persisting in assets).
+        /// </summary>
+        bool StatePersistedHere { get; }
+
         /// <summary>
         /// Time the script was last started
         /// </summary>