bin/ps: Make the rtprio option actually show realtime priorities

bin/ps: Make the rtprio option actually show realtime priorities

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- remove outdated note "101 = not a realtime process" in the man page and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