time: use precise callout for clock_nanosleep(2) and nanosleep(2)

Don't apply tc_precexp and TIMESEL() that uses sbt_timethreshold (both
derivatives of kern.timecounter.alloweddeviation) to sleep callout when
processing the default and precise clocks. The default timer deviation of
5% is our internal optimization in the kernel, and we shouldn't leak that
into the POSIX APIs. Note that application doesn't have any control to
cancel the deviation, only a superuser can change the global tunable [with
side effects].

Leave the deviation for CLOCK_*_FAST and CLOCK_SECOND that are documented
as imprecise.

Provide a sysctl kern.timecounter.nanosleep_precise that allows to restore
the previous behavior.

Improve documentation.

Reviewed by: ziaee, vangyzen, imp, kib
Differential Revision: https://reviews.freebsd.org/D50075