Pass prefer-ipv6 flag through agent config

This builds upon the previous PR adding the prefer-ipv6 environ
config setting. Now, the prefer-ipv6 flag is passed through further
to the agent config, so the agents can initialize the network package
with it even before connecting to the API/state servers (no environ
config yet).
Hehe, pretty descriptive name. I only read the trailing IPv6On as IPv60n and asked myself what it means. ;)