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• remove outdated note "101 = not a realtime process" in the man page and replace it with a more useful reference to man 1 rtprio.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