Ubuntu 16.04 强制使用ipv4安装apt-get

参考文献：https://www.linuxidc.com/Linux/2015-09/123340.htm

起因：每次校园网都要自动用ipv6不知道为什么又总之链接不上，导致安装失败，有以下命令强行切换v6 v4

apt-get全局使用IPv4示例
1.sudo apt-get -o Acquire::ForceIPv4=true install pkg
2.sudo apt-get -o Acquire::ForceIPv4=true update
3.sudo apt-get -o Acquire::ForceIPv4=true upgrade
4.sudo apt-get -o Acquire::ForceIPv4=true dist-upgrade
5.sudo apt-get -o Acquire::ForceIPv4=true install XXX

apt-get全局使用IPv6示例
1.sudo apt-get -o Acquire::ForceIPv6=true install pkg
2.sudo apt-get -o Acquire::ForceIPv6=true update
3.sudo apt-get -o Acquire::ForceIPv6=true upgrade
4.sudo apt-get -o Acquire::ForceIPv6=true dist-upgrade
5.sudo apt-get -o Acquire::ForceIPv6=true install XXX