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/ScriptEngine/Shared/Api/Interface/IOSSL_Api.cs

@@ -178,6 +178,7 @@ namespace OpenSim.Region.ScriptEngine.Shared.Api.Interfaces
         void osNpcSetRot(LSL_Key npc, rotation rot);
         void osNpcStopMoveToTarget(LSL_Key npc);
         void osNpcSay(key npc, string message);
+        void osNpcSit(key npc, key target, int options);
         void osNpcRemove(key npc);
 
         LSL_Key osOwnerSaveAppearance(string notecard);