sched_rr_get_interval writes into the timespec structure pointed
to by tp the round robin time quantum for the process identified by
pid. If pid is zero, the time quantum for the calling process is
written into * tp. The identified process should be running under the
SCHED_RR scheduling policy.
The round robin time quantum value is not alterable under Linux 1.3.81.
POSIX systems on which sched_rr_get_interval is available define
_POSIX_PRIORITY_SCHEDULING in <unistd.h>.