(S corporation shareholders executed buy-sell agreement that restricted transfer of stock shares; buy-sell agreement subsequently amended; Treas. Reg. Sec. 1,1361-1(l)(2)(iii)(A) specifies that buy-sell agreements are to be disregarded in determining whether corporation's outstanding shares of stock confer identical distribution and liquidation rights unless the principal purpose of the agreement is to circumvent the one class of stock requirement and the agreement is entered into at significantly less or more than FMV of stock; no second class of stock created under facts presented).