双币种商城：绑定金锭与非绑定金条的区分消耗

local function buy(actor, price)
    takeitem(actor, "金条", price, 0, "商城")   -- 只扣非绑定，绑定闲置
end

local function buy(actor, price)
    local bound = tonumber(getplayvar(actor, "BIND_GOLD")) or 0
    local useBound = math.min(bound, price)
    local rest = price - useBound
    takeitem(actor, "金锭", useBound, 0, "商城绑定币")
    if rest > 0 then
        takeitem(actor, "金条", rest, 0, "商城非绑定币")
    end
    sendmsg(actor, 1, "绑定扣 " .. useBound .. "，非绑定扣 " .. rest)
end