Open and close listening sockets on localhost:53 depending on if we can

resolve at all.
We come up without listening and open IPv4 and IPv6 sockets once the
resolver process has determinded that we can speak DNS to the outside
world.
Furthermore close the listening sockets when the resolver process
notices that we can no longer speak DNS to the outside.

This is a last-ditch effort to get out of libc resolver's way and let
it speak directly to e.g. dhcp provided resolvers.

With the recomended configuration of having 127.0.0.1 first in
/etc/resolv.conf and then add dhcp provided resolvers after that our
libc resolver will receive a port unreachable error and immediately
switch to the next one if unwind is not listening on port 53.

(Networks have been observed in the wild that intercept DNS packets and
answer NXDOMAIN if an edns0 option is send with the query.)

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