Duchy Adventurers consistently mimic mini-invasion CB mechanics

This change merely makes the Duchy Adventurer CB consistent with
the rest of its mechanics, premise, and purpose. In the event of
the adventurer retinue's [rare] success, they gain any settlements
which they managed to occupy. NOTE: Unlike related invasion CB
mechanics, battle_warscore_mult is unaltered for this CB, making
any occupation beyond more than a county in the target duchy
quite unlikely.