Rewrite of mega-region code to use new form of border checking.

This commit eliminates all of the 'border' class and list code and replaces
it with testing if in the current region.
Impacts: can make a mega-region out of varregions of the same size; and
mega-region combinations must be rectangular (not square but rectangular)

OpenSim/Region/Framework/Interfaces/IRegionCombinerModule.cs

@@ -55,5 +55,10 @@ namespace OpenSim.Region.Framework.Interfaces
         /// Currently, will throw an exception if this does not match a root region.
         /// </param>
         Vector2 GetSizeOfMegaregion(UUID regionId);
+
+        /// <summary>
+        /// Tests to see of position (relative to the region) is within the megaregion
+        /// </summary>
+        bool PositionIsInMegaregion(UUID currentRegion, int xx, int yy);
     }
 }