In the sysclock commit I tried to make 'boottime' a fixed value, but it

In the sysclock commit I tried to make 'boottime' a fixed value, but it
ended up being set to the superblock update time (time of last shutdown)
rather then the real time clock during boot.

Give up on making it a fixed value and just set it to the current time
minus the compensated elapsed time (gd->gd_time_seconds) whenever the
time of day is stepped.  Subsystems which use boottime as an identifier,
such as NFS, already copy it and this change effectively returns boottime
operation to its pre-sysclock algorithm.

Reported-by: YONETANI Tomokazu <qhwt+dragonfly-bugs@les.ath.cx> and others