minetn-6 includes a MAP along with the flags that cause it to be cavernous.
However, the caverns get generated before the MAP is adde the level. Other
similar MAPs have open space on their edges, but there were walls on the
lower right edge of minetn-6. Added another column of mostly open space.
This level can still get cut-off areas, but the stairs won't be located there.
b1fe51e80b