When NICE_WEIGHT * (PRIO_MAX - PRIO_MIN) exceeds the timeshare range,
two CPU-bound threads with different nice values can have the same
priority as their nice values are clamped to the timeshare range limit.
Add static assertion on NICE_WEIGHT to ensure that there is always
enough room for nice values in the both end of the timeshare priority
range.
MFC after: 2 weeks
MFC to: stable/14, stable/15
Sponsored by: FreeBSD Foundation