assault: Improve chance for spies sneaking ashore undetected

assault: Improve chance for spies sneaking ashore undetected

Spies assaulting a foreign sector have only a 10% chance to evade
detection, regardless of efficiency.  With odds like that, players
basically don't bother.

All the other spy detection checks use LND_SPY_DETECT_CHANCE(eff),
which gives 100% spies a 90% chance to evade detection.  That's
perhaps a bit to good here, so let's try LND_SPY_DETECT_CHANCE(eff/2).
A 100% spy now has a 40% chance to sneak ashore undetected.

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