Motivated by the need to schedule retransmits at a granularity less than 1ms while not swamping the CPU with excess timer interrupts.
- change ticks to long
- add a separate hardclock frequency: kern.hardclock_hz
- make callout precision behave sensibly, where value passed is 0 or less than SBT_1US it gets replaced with sbt corresponding to hardclock_hz (SBT_1MS by default)
- add tvtohz64
- replace calls likely to wrap at higher frequency with tvtohz64