Allow all blocks that get the IGN_PASSABLE flag set but which the player
can stand on to be ground from height 0 on (GROUND_HEIGHT, takes
effect unless for blocks with an explicit workaround). Set IGN_PASSABLE
and GROUND_HEIGHT for ENDER_PORTAL_FRAME explicitly.
Seems better to have a dedicated module for this as well, since other
modules might be built against different versions of the Bukkit-API,
potentially.
