Adds preconditions to systemd unit file to run named-checkconf before a restart or reload.
Without this check, it is easy to miss small typos when editing zone files, and when you do that, there is no obvious error in response your "systemctl restart" or "systemctl reload" command. (The failure is recorded in the log.)
With this check, systemctl outputs an error message if the check fails (predicting that the restart or reload command would fail), making mistakes easier to notice.