systemd: Add "After" dependency on time-sync.target

systemd: Add "After" dependency on time-sync.target

Adds 'After=time-sync.target' to the service unit files; this will not
have any immediate effect, unless the system administrator has also
enabled a unit which has a 'Before' dependency on that target;
there are none of those units enabled in a default configuration in
common distributions. systemd itself adds such an 'After' dependency
to all timer units with OnCalendar settings, and to some other units.

However, in a system which makes use of systemd-timesyncd, the admin
can enable systemd-time-wait-sync.service, which has such a
dependency. The result would be that none of the PowerDNS services on
the system would be started until after systemd-timesyncd is certain
that the system's clock is synchronized with the configured external
source.

A similar configuration can be achieved on systems which use ntpd,
chronyd, or other time synchronization tools.

Closes #11153.