Save real-time clock on reboot

For computers without a real-time clock, this utility enables to store
the wall-clock on shutdown and restore it on boot, in order to minimize
the drift as far as possible until e.g. NTP is used to set the correct
date/time again.

Note that systems using UFS for the root fs won't need this, as the
clock is already restored from info in the UFS superblock.
Since __FreeBSD_version 1600020 ZFS also restores the wall-clock time
from the rootfs.
https://cgit.freebsd.org/src/commit/?id=d0b3ecdc274930e190ea233b6b69ff03782eaf8d