Make changes to IPv6 pool to add forwarding information

We shouldn't be using ifconfig (deprecated). We also should be removing ip addr add from this, because the changes won't be saved permanently unless they're in the /etc/network/interfaces file (or similar, depending on distribution).

I've kept the second IPv6 address add section, but removed the first because it's unnecessary to have the same information twice in the same documentation. 

While ArchLinux uses systemd-networkd now, there's a lot of people still using netctl, so I believe that we'll be okay keeping netctl in there, but will likely add systemd-networkd as well shortly. This change is more of a "pushing it out there because adding IPv6 by ip addr add is a bad idea" kind of change. 

Additionally, I've added information about IPv6 forwarding. While I don't want to add information about IPv6 forwarding at a kernel/sysctl level (eg. making your Linode a router, essentially), I do want to add a bit about /116 pools.