Should wg-quick with SaveConfig accumulate IPv6 link-local addresses

[Jason A. Donenfeld]

This is mostly just a CentOS issue. On older kernels, we don't
successfully disable LLv6:
https://git.zx2c4.com/WireGuard/tree/src/compat/compat.h#n583

There's probably a way to backport that tweak to before 3.17, but I
haven't spent time on it. Feel free to send a patch, and if it's clean
and fits entirely into compat.h, I'll gladly apply it.