We don't want to have to wait for the timer to expire for the updater
to make its first run. This adds a timer unit which triggers the
geoipupdate.service unit immediately, but only runs if the configured
DatabaseDirectory doesn't exist yet.
The database directory needs to be created before the
geoipupdate.service unit is activated; otherwise, systemd will not be
able to set up the mount namespacing to grant the service read-write
access.
