Add natives to list zones in other plugins

MapZone_GetGroupZones, MapZone_IsClusteredZone and
MapZone_GetClusterZones can be used to get the names of all zones and
clusters in the group and zones in a cluster.
Fix clients not leaving zones on death

trigger_multiple don't count players as leaving when they die within a
zone.
We guarantee that the leave callback is called when players die.