Screen erasure leaves the map set to spaces. If S_unexplored is
something other than <space>, tty wasn't drawing with S_unexplored
after a menu or long message line got erased following temporary
overwrite of part of the map.
This seems to work but is not the correct way to do things.
clear_screen(), cl_eos(), and cl_end() should all be taught to
flag the map as needing to be refreshed after they erase part of it.
tty_clear_nhwindow(WIN_BASE) is also lacking since it erases the
message line, full map, and status lines but leaves their internal
windows with stale data about what is shown instead of marking them
blank.
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