This is needed when unbound config doesn't set "do-daemonize: no" by itself otherwise starting service fails with:
systemd[1]: unbound.service: Got notification message from PID <PID>, but reception only permitted for main PID which is currently not known
https://github.com/NLnetLabs/unbound/blob/release-1.9.3/doc/example.conf.in#L236
[Service]
ExecReload=/bin/kill -HUP $MAINPID
-ExecStart=@UNBOUND_SBIN_DIR@/unbound
+ExecStart=@UNBOUND_SBIN_DIR@/unbound -d
NotifyAccess=main
Type=notify
CapabilityBoundingSet=CAP_IPC_LOCK CAP_NET_BIND_SERVICE CAP_SETGID CAP_SETUID CAP_SYS_CHROOT CAP_SYS_RESOURCE CAP_NET_RAW