initscript: Try harder to synchronize the clocks

Most of the time, the NTP client manages to synchronize the clocks within 2 seconds so I originally set a timeout of 5 seconds just to be sure this would be enough everywhere. However, it was sometimes failing, so I bumped it to 10s... which is apparently still not working...

I am now suspecting that we would be better off killing the ntp client and retrying up to 3 times, with a 5 seconds timeout rather than bumping to 15s. This commit does just that :)

Closes: #32 (moved)