clear userAgent state on client shutdown, which fixes the issue

where you could only login once with a given id in standalone mode.

OpenSim/Framework/Data/UserData.cs

@@ -84,6 +84,12 @@ namespace OpenSim.Framework.Data
         /// <param name="user">UserProfile to add</param>
         void addNewUserProfile(UserProfileData user);
 
+        /// <summary>
+        ///  Updates an existing user profile
+        /// </summary>
+        /// <param name="user">UserProfile to update</param>
+        bool updateUserProfile(UserProfileData user);
+
         /// <summary>
         /// Adds a new agent to the database
         /// </summary>