Restore [Startup] physical_prim flag which can stop any prims being subject to physics

This had stopped working.  However, at the moment it still allows the physics flag to be set even though this has no effect.  This needs to be fixed.
Default for this flag is true as previously.

OpenSim/Framework/ConfigSettings.cs

@@ -34,7 +34,13 @@ namespace OpenSim.Framework
         public bool   See_into_region_from_neighbor { get; set; }
         public string StorageDll                    { get; set; }
         public string ClientstackDll                { get; set; }
+
+        /// <summary>
+        /// Controls whether physics can be applied to prims.  Even if false, prims still have entries in a
+        /// PhysicsScene in order to perform collision detection
+        /// </summary>
         public bool   PhysicalPrim                  { get; set; }
+        
         public string LibrariesXMLFile              { get; set; }
 
         public const uint DefaultRegionHttpPort = 9000;