freshly extended trade file makes sense. When set() removes a lot,
simply zero the slot.
(set): Initialize trd_uid.
(trade_check_ok): Use check_land_ok() & friends instead of doing it by
hand, but wrong. This fixes bying planes and land units loaded on
ships that moved after loading.
(check_trade_ok): New.
(trade_check_ok): Use it. Remove unused argument lot. Callers
changed.