Add duplicator.Disallow (#1676)
Used when you want to override the default duplication behaviour, eg when an
Addon makes itself duplicatable but you don't want it to be.
Also useful if you want a script such as persistence to be able to duplicate
things that players cannot.
```lua
duplicator.Allow( "prop_door_rotating" )
local tab = duplicator.CopyEnts( Ents )
duplicator.Disallow( "prop_door_rotating" )
```