pthread_attr_setinheritsched.3, pthread_attr_setschedparam.3, pthread_attr_setschedpo...

pthread_attr_setinheritsched.3, pthread_attr_setschedparam.3, pthread_attr_setschedpolicy.3, pthread_setaffinity_np.3, pthread_setschedparam.3, pthread_setschedprio.3, pthread_yield.3, pthreads.7: Change references to "sched_setscheduler(2)" to "sched(7)"

Change consistent with the fact that the scheduling overview
page is now sched(7) not sched_setscheduler(2).

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>