getpgid,getsid: Add capability mode checks

[ojanerif_amd.com]

The same pattern exists in sched_{get,set}param, sched_{get,set}scheduler and sched_rr_get_interval (p1003_1b.c), and in rtprio(2)/rtprio_thread(2). The latter uses tdfind(lwpid, -1), so any TID on the system can be probed. I put up a separate review for those in D61412.