Move common code to detect whether a part has a valid sit target into a SOP property rather than being repeated in SP.

This also makes the detection in SP.FindNextAvailableSitTarget() and SendSitResponse() identical.
Previously they varied slightly (SendSitResponse didn't check for an older type of invalid quaternion) but the practical effect is most probably zero.

OpenSim/Region/Framework/Scenes/SceneObjectPart.cs

@@ -135,6 +135,21 @@ namespace OpenSim.Region.Framework.Scenes
             get { return ParentGroup.RootPart == this; } 
         }
 
+        /// <summary>
+        /// Is an explicit sit target set for this part?
+        /// </summary>
+        public bool IsSitTargetSet
+        {
+            get
+            {
+                return
+                    !(SitTargetPosition == Vector3.Zero
+                      && (SitTargetOrientation == Quaternion.Identity // Valid Zero Rotation quaternion
+                       || SitTargetOrientation.X == 0f && SitTargetOrientation.Y == 0f && SitTargetOrientation.Z == 1f && SitTargetOrientation.W == 0f // W-Z Mapping was invalid at one point
+                       || SitTargetOrientation.X == 0f && SitTargetOrientation.Y == 0f && SitTargetOrientation.Z == 0f && SitTargetOrientation.W == 0f)); // Invalid Quaternion
+            }
+        }
+
         #region Fields
 
         public bool AllowedDrop;