In survival mode, zoom is disabled, the binoculars item is needed to
allow a zoom with a 10 degree FOV, realistic for compact binoculars.
Creative mode or per-player creative privilege allows a zoom with a
15 degree field of view (the default MT engine zoom FOV).
Uses the new player object property 'can zoom'.
In survival mode, use of zoom requires the binoculars item in
inventory.
Zoom is automatically allowed in creative mode and for players with
the 'creative' privilege.
The binoculars.update_player_property() function is global so can be
redefined by a mod for alternative behaviour.
