Mantis#1207. Thank you, TGlion for a patch that addresses:

Implementation of llModifyLand() and There is a bug on 
permission-check of land-terraforming: x an y-coordinates 
are interchanged on function-call ExternalChecksCanTerraformLand.
Correct: x is west, and y is north. 2) Missing check of 
"Other allow to terraform-flag" (Parcel.ParcelFlags.AllowTerraform)

OpenSim/Region/Environment/Modules/World/Terrain/ITerrainModule.cs

@@ -25,7 +25,9 @@
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+
 using System.IO;
+using OpenMetaverse;
 
 namespace OpenSim.Region.Environment.Modules.World.Terrain
 {
@@ -33,7 +35,8 @@ namespace OpenSim.Region.Environment.Modules.World.Terrain
     {
         void LoadFromFile(string filename);
         void SaveToFile(string filename);
-
+        void ModifyTerrain(Vector3 pos, byte size, byte action, UUID agentId);
+        
         /// <summary>
         /// Load a terrain from a stream.
         /// </summary>