decouple maximum clock frequency from callout scheduling granularity

[kmacy]

Summary

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

[davide]

The changes look good. I'd split them in multiple patches/commit, more or less following the structure you outlined in the summary, but thanks for sending out a single one so we can have the whole picture.
IIRC there were objections to change ticks from 32-bits to 64-bits so you may want to ask feedback from a more general audience.
Other than that, I'm fine with (a splitted version of this) getting in.

sys/kern/kern_clock.c

472

Atomic on 64-bit integers are slightly more expensive than the 32-bit counterparts. I think it's fine, but do we have numbers on what's the impact of this? (maybe profiling).

sys/kern/kern_tc.c

1285

I may miss something here, but do you ever use the value of the counter you just read?

sys/kern/kern_timeout.c

597

Please add a comment specifying why you round-up, for clarity.

957

Can this be KASSERT() maybe with a message attached "precision needs to be at least 1US"? I'm not fussed about it, so if you don't like it you can leave it as is.

sys/kern/subr_param.c

88

I think you ate a whitespace two lines below.

116

Nit: SYSCTL descriptions always start with capital letters, can you change that?

sys/sys/callout.h

61

Nit again, this seems unaligned.