Eyes plus stinking cloud

Eyes plus stinking cloud
stinking clouds extend their timers, causing the "ttl == 0" check in
visible_region_at to be inappropriate; technically it was never quite
right, since the ttl is set to 0 one turn before removal is considered. But
with the Eyes on, this caused a visible change in the region although the
region still existed.  Introduced a new -2L value to designate that the
region is being removed (-1L means it's permanent), which is what
visible_region_at was really trying to test.