gthr-posix.h: Include <unistd.h> for feature tests.

* gthr-posix.h: Include <unistd.h> for feature tests.
(sched_get_priority_max, sched_get_priority_min)
(pthread_getschedparam, pthread_setschedparam): Only use
if _POSIX_THREAD_PRIORITY_SCHEDULING is defined.
(__gthread_objc_thread_set_priority): Don't treat all non-zero
returns from sched_get_priority_max and sched_get_priority_min
as an error.

From-SVN: r58630