RTP_LOOKUP threw for every thread made with the sce attributes, which
allow only SCHED_FIFO and SCHED_RR, since the rtprio mapping was
unmeasured. In a payload process on a PS5 (firmware 12.70) the lookup
reports type 10 for SCHED_FIFO and 2 for SCHED_RR with prio equal to the
scheduling priority, for the calling thread and for another live thread
by its id; an unknown id fails with ESRCH, and so did function 2. Report
that, and keep RTP_SET throwing for real-time and other threads, where no
accepted value was measured.