Fix the rtprio option that for some reason was progessively becoming an
option showing the priority class of threads. In particular:
- use the constants defined in sys/sys/rtprio.h instead of those defined in
sys/sys/priority.h: this helps making clearer that the code actually is
about realtime priorities and not standard scheduler priorities;
- remove the PRI_ITHD case that has nothing to do with realtime priorities;
- convert the priority levels to realtime priority levels using the same
formulas used for pri_to_rtp function in sys/kern/kern_resource.c.
- remove outdated note "101 = not a realtime process" in the man page and
replace it with a more useful reference to man 1 rtprio.
Approved by: src (mckusick), manpages (bcr), gerald (mentor)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D25266
50301bb0a1