realtime priorities
The current `ps -axO rtprio' show threads running at interrupt
priority such as the [intr] thread as '1:48' and threads running
at kernel priority such as [pagedaemon] as normal:4294967260.
This change shows [intr] as intr:48 and [pagedaemon] as kernel:4.
Reviewed by: kib
MFC after: 1 week (together with -r362369)
Differential Revision: https://reviews.freebsd.org/D25660
4a022f0e06