openntpd: add SysV initscript and user

Add the _ntp user that the daemon requires to work in any way.
Add a SysV-style initscript as well, but not with '-s' (set time
immediately after startup) because it can make the boot process stall
for a few seconds if there's no proper network connectivity/dns setup.
Make ntp and openntpd mutually exclusive since they overstep each other.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>