#326 disable ability to buy items from merchant

This will disable the 'buy' option of an item on a merchant if in the item script:

function buy_display_flags(Item, Spawn)
    return 128
end

same can be done with buyback items:

function buyback_display_flags(Item, Spawn)
    return 128
end

based on these flags, as far as I know other than no buy and no buyback (why would you use no buyback on the main buy page I don't know).. are the only ones available. Rest are for the sales tab

#define DISPLAY_FLAG_RED_TEXT           1 // old clients
#define DISPLAY_FLAG_NO_GUILD_STATUS    8
#define DISPLAY_FLAG_NO_BUYBACK         16
#define DISPLAY_FLAG_NOT_FOR_SALE       64
#define DISPLAY_FLAG_NO_BUY             128 // disables buying on merchant 'buy' list