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/Framework/IClientAPI.cs

@@ -61,7 +61,7 @@ namespace OpenSim.Framework
 
     public delegate void ModifyTerrain(
         float height, float seconds, byte size, byte action, float north, float west, float south, float east,
-        IClientAPI remoteClient);
+        UUID agentId);
 
     public delegate void SetAppearance(byte[] texture, List<byte> visualParamList);