KeepInvZone

1.1 was not working properly, now everything works (hopefully?)

Modified commands:

/setkeepinvzone is now /keepinvzone set

/remkeepinvzone is now /keepinvzone remove

Added command:

/keepinvzone list displays the list of created zones and their coordinates.

Fixes:

Previously, when dying inside a zone, items were dropped on the ground and also restored in the inventory, causing duplication. This no longer happens. When death occurs inside a zone, no items are dropped.
Works now in singleplayer, not sure why anyone would need that, but it works.