Enable IPv6 with Raspberry Pi

[noraworld]

• https://ktaka.blog.ccmp.jp/2020/05/linuxipv6-ipoe.html
• https://inaz2.hatenablog.com/entry/2016/04/19/010458

[noraworld]

I tried asking the question, but I couldn't resolve it.

• https://forum.openwrt.org/t/how-to-install-odhcpd/140204
• https://teratail.com/questions/ish4s908glivxm