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).
When testing the API we write the acronym in upper-case. So maybe a localAPIAddr would be correct here.