des.region() accepted booleans for the joined field, whereas des.room
accepted xchars. These were only being used as truth values, so this
converts the room ones into booleans for consistency. I don't think
accidentally using an int or a boolean wrongly would actually crash the
level generator, but consistency is good.
This converts an schar field in struct mkroom into a boolean; on most
systems these are probably 1-byte types and save files won't be broken,
but it might be best to treat this as a save breaker anyway.
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