Convert room 'joined' and 'needjoining' into booleans

Convert room 'joined' and 'needjoining' into booleans

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.