Implement osNpcSit(). This is still in development so don't trust it

Format is osNpcSit(<npc-uuid>, <target-uuid>, OS_NPC_SIT_IMMEDIATE)
e.g. osNpcSit(npc, llGetKey(), OS_NPC_SIT_IMMEDIATE);
At the moment, sit only succeeds if the part has a sit target set.
NPC immediately sits on the target even if miles away - they do not walk up to it.
This method is in development - it may change so please don't trust it yet.
Standing will follow shortly since that's kind of important once you're sitting :)

OpenSim/Region/Framework/Interfaces/INPCModule.cs

@@ -94,6 +94,15 @@ namespace OpenSim.Region.Framework.Interfaces
         /// <returns>True if the operation succeeded, false if there was no such agent or the agent was not an NPC</returns>
         bool Say(UUID agentID, Scene scene, string text);
 
+        /// <summary>
+        /// Sit the NPC.
+        /// </summary>
+        /// <param name="agentID"></param>
+        /// <param name="partID"></param>
+        /// <param name="scene"></param>
+        /// <returns>true if the sit succeeded, false if not</returns>
+        bool Sit(UUID agentID, UUID partID, Scene scene);
+
         /// <summary>
         /// Delete an NPC.
         /// </summary>