FreeBSD Port: openntpd-4.6_1,2

[f5b]

openntpd run longtime(about 60 days) in FreeBSD 8.1-RELEASE and system time
is accurate.
but when reboot the system, the time will become inaccurate (diff about 10
minutes of GMT), and will have some day to become accurate through
synchronizing pool.ntp.org

but I heard ntpd in FreeBSD 8.1-RELEASE will never happen things like
openntpd.
when reboot the system, time is always accurate because ntpd had write time
back to RTC In 30 minutes.

r207360，r207359, r207362, callout

reference: r207360，r207359, r207362, callout
https://blog.delphij.net/2010/08/rtc-ntp.html

sorry for my pool English.