说明：  程序，要求程序运行后进入一个无限循环，打印用户按下的键的值，当用户按下中断键(Ctrl+Z)时，进入程序的自定义信号处理函数，当用户再次按下中断键(Ctrl+Z)后，结束程序运行；
(Procedures, requirements to run the program into an infinite loop, printing the user presses a key value, when the user presses the interrupt key (CTRL+ Z), custom signal processing functions into the program when the user again, press the interrupt key (Ctrl+ Z) after the end of the program is running )