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/Scenes/SceneObjectPart.cs

@@ -1220,14 +1220,12 @@ namespace OpenSim.Region.Framework.Scenes
             }
         }
 
-        
         public UUID SitTargetAvatar
         {
             get { return m_sitTargetAvatar; }
             set { m_sitTargetAvatar = value; }
         }
 
-        
         public virtual UUID RegionID
         {
             get