When blocked on a mutex, display the mutex name via the wchan string field

so we can at least tell the difference between being blocked in Giant
and being blocked in some other mutex.

bin/ps/print.c

@@ -407,8 +407,11 @@ wchan(KINFO *k, VARENT *ve)
 		else
 			(void)printf("%-*lx", v->width,
 			    (long)k->ki_p->ki_wchan);
-	} else
+	} else if (k->ki_p->ki_kiflag & KI_MTXBLOCK) {
+		(void)printf("%-*.*s", v->width, v->width, k->ki_p->ki_mtxname);
+	} else {
 		(void)printf("%-*s", v->width, "-");
+	}
 }
 
 #ifndef pgtok