create raw socket for icmp protocol and drop root privileges if running setuid.

编写一个简单的 ping 程序。利用 Raw Socket 封装和发送以太网帧的功能，实现 ICMP 包的发送和接收，并输出结果。(Write a simple ping program. Using Raw Socket encapsulation and send Ethernet frame function, to achieve ICMP packet sending and receiving, and output the results.)

基于RAW套接字的ICMP协议ping功能源码，linux平台验证通过。(Based on RAW socket's ICMP protocol ping function source, Linux platform verification is adopted.)