a number of Subjects files to prevent name
conflicts where the only the case is difference.
Attack -> Attacking
Moving -> Transportation
Move -> Moving
Market -> Commerce
Nation -> Nations
Production -> Producing
Update -> Updating
display_mark() to callers. buy() no longer accepts "all". It used to
display all lots, but didn't let you buy any. Similar nonsense
happened for "" if buy() prompted for it.
(display_mark): Fix size of cheapest_items[].
(player_coms): Document mark accepting "all".
