MINOR: sock-inet: detect apparent IPv6 connectivity

MINOR: sock-inet: detect apparent IPv6 connectivity

In order to ease dual-stack deployments, we could at least try to
check if ipv6 seems to be reachable. For this we're adding a test
based on a UDP connect (no traffic) on port 53 to the base of
public addresses (2001::) and see if the connect() is permitted,
indicating that the routing table knows how to reach it, or fails.
Based on this result we're setting a global variable that other
subsystems might use to preset their defaults.