Encounter v1.5 Trading with trader

Now, player is able to trade with another trader upon encounter. Price
is calculated not based on tech level (universe does not have tech
level, of course). player's trader skill gives a trading discount. the
discount is calculated by (100-sp)%. So, I assume the trader sp will
never exceed 100. Also, discount is applied for both buying and selling
(even though, it does not sound reasonable). While in a fight, if player
surrendered, trader would get every good that player owns, but leave
some money for player. (Surrender is now only working for the fight with
traders).