openntpd: set time immediately from initscript

Match the systemd service file and set time immediately on startup
rather than small steps when it differs a lot.
On embedded scenarios this is better since boards that lack a
battery-backed RTC might start at unix epoch and the time set will delay
for quite a while otherwise.
For boards that do have a battery-backed RTC the behaviour will be
practically the same unless the RTC drifts a lot.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>