Compensate for allow-nether/allow-end as false; BUKKIT-3466

When either of those settings are false, the worlds are not loaded and
therefore will not be targeted for portal exits.  Existing worlds are
iterated directly to avoid defaulting to the first world if a direct
dimension match is not found.

Plugins must also specify exit from custom Bukkit worlds to comply with
original commit: https://github.com/Bukkit/CraftBukkit/commit/2dc2af0

This commit introduces a constant to clarify the dependency on the
CraftBukkit implementation of custom worlds having a dimension offset.