Add mac support

[FlowerWrong]

According this pr, water auto add or remove {0,0,0,2} on mac now. I have added mac support. The only problem is that the DNS server must be 127.0.0.1 or your ip address. If you want to set to the tun address, I think we can add a nat, like 10.192.0.1:53 -> 10.192.0.1:5353.

[xjdrew]

请问有在mac上测试过么，测试方式是怎样的，能正常工作吗？

[FlowerWrong]

Mac下测试过，日常使用。用了多久我忘了，不少于两周。开始有个下载大文件中断的问题，我就没用了。后来在v2ex有网友指出了问题，目前主力开发机换linux，现在linux下长期使用。