Permit timed sleeps before timers are working.

[jhb]

Summary

Permit timed sleeps before timers are working.

The callout subsystem already handles early callouts and schedules
the first clock interrupt appropriately based on the currently pending
callouts.

Test Plan

• I booted kernels both with and without EARLY_AP_STARTUP with the included test callouts (which I will remove for the actual commit).
  
  One oddity is that the timers actually fire about 1 second too early. That is, sbinuptime() is about 1 second when the test SYSINIT runs, but the time base that the delta is added to is pulled from DPCPU_GET(hardclocktime) which is 0. So sample output:

CALLOUT: uptime at test start: 4294988770
CALLOUT: time when starting: 4295388202
CALLOUT: 1 second fired at 5417817195
CALLOUT: 2 second fired at 8671043041
CALLOUT: 4 second fired at 17283412636
CALLOUT: 10 second fired at 42952326079

If you examine the values in hex you get 0x1000053e2 for the test start
and 0x100066c2a when the timers start.  The first callout fires about a
quarter of a second later at 0x142ed546b.  The remaining callouts are
all about a second early: 0x204d59de1, 0x4062bf29c, and 0xa00287bbf.

I could "fix" this by patching callout_when() to use sbinuptime() as
the time base instead of DPCPU_GET() before timer interrupts are
initialized, or we could just let it be?

Also, I had originally suggested to still panic in the sleepq code
for thread0, so if that is desired I can restrict the panic that this
removes to only be for thread0.

Finally, callout_reset() that is too early should panic due to
cc_inited == 0 check in callout_reset_sbt_on(), so need to add a new
check for that.

[kib]

My current feel is that it is fine to leave the 1 sec inaccuracy as is, until proven harmful. As I understand the current state, early callouts are guaranteed to fire not earlier than timer starts, which make the attempt to handle this rather ineffective.

OTOH, I do think that panic for thread0 sleeping too early is the good thing to have, since this is the feature that would allow to detect deadlocks with non-functional callouts required to make the progress.

[jhb]

• Bring back the timed sleep panic, but only for thread0.
• Handle callouts scheduled before DPCPU_GET(hardclocktime) is set.
• Print times in hex.

[jhb]

Latest output (now in hex since that is simpler to work with):

CALLOUT: uptime at test start: 1000064a9
CALLOUT: time when starting: 10007570c
CALLOUT: 1 second fired at 236c02328
CALLOUT: 2 second fired at 32214bee0
CALLOUT: 4 second fired at 500287315
CALLOUT: 10 second fired at b162dd900

sys/kern/kern_timeout.c

1009

This proved simpler to fix than I had imagined. This fixes "off by 1 second" problem.

[jhb]

Forgot to include reference in commit rS309148.