Update to D25266, bin/ps: Make the rtprio option actually show

Update to D25266, bin/ps: Make the rtprio option actually show
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