clockintr: move hz(9)-based initialization out to initclocks()

clockintr: move hz(9)-based initialization out to initclocks()

To separate the hardclock from the clock interrupt subsystem we'll
need to move all related state out first.

hz(9) is set when we return from cpu_initclocks(), so it's safe to
move hardclock_period and roundrobin_period initialization out into
initclocks().  Move hardclock_period itself out into kern_clock.c
alongside the statclock variables.