Again, thanks to Alondria for:

Added: LinkNum to SceneObjectPart
Added: Bunch-o settings of LinkNum in SceneObjectGroup
Added: llGetNumberOfPrims()
Added: llGetLinkNumber()
Added: llGetLinkKey()
Added: llGetLinkName() (and change to string return type)

OpenSim/Region/Environment/Scenes/SceneObjectPart.cs

@@ -364,6 +364,13 @@ namespace OpenSim.Region.Environment.Scenes
             set { m_touchName = value; }
         }
 
+        private int m_linkNum = 0;
+        public int LinkNum
+        {
+            get { return m_linkNum; }
+            set { m_linkNum = value; }
+        }
+
         private byte m_clickAction = 0;
         public byte ClickAction
         {