update/revolt: Don't kill land units without military

update/revolt: Don't kill land units without military

Land units without military can't contribute to the fight.  They can
still get killed, and whether they are depends on their UID.

take_casualties() kills land units in UID order until the required
number of casualties is reached.  Killing a land unit without military
provides none, but take_casualties() doesn't care.  The land unit
"dies fighting guerrillas", which makes no sense when it's doesn't
have any military.

If the rebels win, they attempt to capture any surviving land units.
Spies hide or get executed instead.  Same as for any other violent
sector takeover.

Normal ground combat ignores land units without military.  Do the same
here: ignore them in take_casualties().  This protects spies and other
land units without military from the fighting, but exposes them to
capture.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>