update/revolt: Destroy land units only when casualties demand it
take_casualties() first applies casualties without destroying land
units, and if any remain, applies them by destroying land units. This
is wrong, because the remaining casualties may not suffice to actually
kill. Has been that way since land units were added in Chainsaw 3.
Apply remaining casualties more carefully: destroy land units only
when efficiency falls below 10%.
Signed-off-by: Markus Armbruster <armbru@pond.sub.org>