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- remove outdated note "101 = not a realtime process" in the man page and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

bin/ps/print.c

@@ -704,17 +704,17 @@ priorityr(KINFO *k, VARENT *ve __unused)
 	class = lpri->pri_class;
 	level = lpri->pri_level;
 	switch (class) {
-	case PRI_ITHD:
-		asprintf(&str, "intr:%u", level);
+	case RTP_PRIO_REALTIME:
+	/* alias for PRI_REALTIME */
+		asprintf(&str, "real:%u", level - PRI_MIN_REALTIME);
 		break;
-	case PRI_REALTIME:
-		asprintf(&str, "real:%u", level);
+	case RTP_PRIO_NORMAL:
+	/* alias for PRI_TIMESHARE */
+		asprintf(&str, "normal:%u", level - PRI_MIN_TIMESHARE);
 		break;
-	case PRI_TIMESHARE:
-		asprintf(&str, "normal");
-		break;
-	case PRI_IDLE:
-		asprintf(&str, "idle:%u", level);
+	case RTP_PRIO_IDLE:
+	/* alias for PRI_IDLE */
+		asprintf(&str, "idle:%u", level - PRI_MIN_IDLE);
 		break;
 	default:
 		asprintf(&str, "%u:%u", class, level);

bin/ps/ps.1

@@ -648,7 +648,8 @@ group name (from rgid)
 .It Cm rss
 resident set size
 .It Cm rtprio
-realtime priority (101 = not a realtime process)
+realtime priority (see
+.Xr rtprio 1)
 .It Cm ruid
 real user ID
 .It Cm ruser