do not differentiate manually configured address from autoconfigured ones wrt prefix management; - always earn a reference to the prefix when an address is configured  (by ioctl). - always delete the prefix when an address that has the last referene   is manually removed.
do not differentiate manually configured address from autoconfigured ones
wrt prefix management;
- always earn a reference to the prefix when an address is configured
 (by ioctl).
- always delete the prefix when an address that has the last referene
  is manually removed.

The change should solve the problem raised in KAME-snap 6989.

sync w/kame

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars