remove obsoleted parts of setup/dns.sh

Now that dns_update is a part of the management daemon, we no
longer are using STORAGE_ROOT/dns for anything.
two more cases of shelling out external programs in a more secure way, see cecda9cec5643d7b89829d1663fcedea346eca23
write nsd.conf zones in a predictable order so that we don't keep rewriting it