MINOR: resolvers: use the runtime IPv6 status instead of boot time one

MINOR: resolvers: use the runtime IPv6 status instead of boot time one

On systems where the network is not reachable at boot time (certain HA
systems for example, or dynamically addressed test machines), we'll want
to be able to periodically revalidate the IPv6 reachability status. The
current code makes it complicated because it sets the config bits once
for all at boot time. This commit changes this so that the config bits
are not changed, but instead we rely on a static inline function that
relies on sock_inet6_seems_reachable for every test (really cheap). This
also removes the now unneeded resolvers late init code.

This variable for now is still set at boot time but this will ease the
transition later, as the resolvers code is now ready for this.