No bug yet: allow sched_setscheduler, used by audio thread in e.g. CubeVid.

It might be possible to restrict this syscall more -- in the case I saw,
arg 0 was the caller's tid and arg 1 was SCHED_OTHER; i.e., we could
block trying to affect other threads and the use of realtime policies,
if we're not legitimately doing that anywhere.  (We can't inspect the
struct sched_param passed by pointer, however.)