Divide-by-zero panic via kern.hz=0 loader tunable

Summary

hz fetched from kern.hz with no >0 check(:200), used as divisor 3x(:204 ustick,:205 nstick,:207 ntp_default_tick_delta=howmany(30000000,60*hz)). kern.hz=0 at loader -> synchronous divide exception on boot. Boot-time DoS only.