Added a (xmlIgnored) SitAnimation property to SceneObjectPart. That allows the setting of the name of the animation to be used when a avatar sits on that object. At some point in the future this should be persisted.

So basically simplifies what a lsl script that detects a avatar sitting on a prim, then stopping the sit animation and playing a custom animation, does.
Also added another ScenePresence.HandleAgentRequestSit() method , that accepts the name of the sit animation. So that modules can override the animation used, when they are doing a server controlled sit.
Started some work on making the stand pose be played as soon as a user logs into a region. Rather than them starting with their arms stretched. This still needs more work

OpenSim/Region/Environment/Scenes/SceneObjectPart.cs

@@ -160,6 +160,7 @@ namespace OpenSim.Region.Environment.Scenes
         private string m_sitName = String.Empty;
         private Quaternion m_sitTargetOrientation = Quaternion.Identity;
         private Vector3 m_sitTargetPosition = Vector3.Zero;
+        private string m_sitAnimation = "SIT";
         private string m_text = String.Empty;
         private string m_touchName = String.Empty;
         private readonly UndoStack<UndoState> m_undo = new UndoStack<UndoState>(5);
@@ -1017,6 +1018,13 @@ if (m_shape != null) {
             set { _parentUUID = value; }
         }
 
+        [XmlIgnore]
+        public string SitAnimation
+        {
+            get { return m_sitAnimation; }
+            set { m_sitAnimation = value; }
+        }
+
         #endregion Public Properties with only Get