Rewrite subr_sleepqueue.c use of callouts to not depend on the
specifics of callout KPI. Esp., do not depend on the exact interface
of callout_stop(9) return values.

The main change is that instead of requiring precise callouts, code
maintains absolute time to wake up. Callouts now should ensure that a
wake occurs at the requested moment, but we can tolerate both run-away
callout, and callout_stop(9) lying about running callout either way.

As consequence, it removes the constant source of the bugs where
sleepq_check_timeout() causes uninterruptible thread state where the
thread is detached from CPU, see e.g. r234952 and r296320.

Patch also removes dual meaning of the TDF_TIMEOUT flag, making code
(IMO much) simpler to reason about.

Tested by: pho
Reviewed by: jhb
Sponsored by: The FreeBSD Foundation
MFC after: 1 month
Differential revision: https://reviews.freebsd.org/D7137