Boost thread priority while changing CPU frequency

Boost the priority of user-space threads when they set
their affinity to a core to adjust its frequency. This avoids a situation
where a CPU bound kernel thread with the same affinity is running on a
down-clocked core, and will "block" powerd from up-clocking the core
until the kernel thread yields. This can lead to poor perfomance,
and to things potentially getting stuck on Giant.

Reviewed by: kib (imp reviewed earlier version)
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D15246