modernize systemd.service file

Turns out that our systemd.service file is from the age of dinosaurs.

Looking at https://gitlab.isc.org/isc-private/rpms/bind/-/blob/5b649ff34e7047ac4be6c82993c6f9784958533c/named.service.in (named.service.in):

• type should be notify-reload
• ExecReload, ExecStop, PIDFile commands should go away

Pros: This will provide sensible integration and detection when BIND is ready (including reload) - at least for the simple cases where no catalog zones or RPZ is involved.

It might also make debugging a bit easier as tighter integration should avoid corner cases around kill invocation.

Edit: fixed the link.