assault: Don't send spies into combat

assault: Don't send spies into combat

The server doesn't let you send land units without offensive strength
into combat: ask_olist() simply doesn't offer them.  Good.

However, it needs to offer spies for assault, because assault is how
they sneak ashore.  To make it offer spies, which have no offensive
strength, attack_val() artificially sets their offensive strength to
one for assault.  Dirt effect: spies fight (and die) in assaults, even
though they can't otherwise attack.  Lame.  Has been that way since
spies were added in 4.0.0.

Make ask_olist() offer spies regardless of offensive strength when
assaulting, and drop the special case from attack_val().  They get
offered exactly as before.  However, since their offensive strength is
now zero, they won't enter actual combat (see the previous commit).

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