Rename cpuset_t to cpu_set_t which is the type used in some non-standard...

Rename cpuset_t to cpu_set_t which is the type used in some non-standard Linux/GNU interfaces.  Move definitions of cpu_set_t to include/sys/types.h.  Add prototypes for sched_setaffinity(), sched_getaffinity(), pthread_attr_setaffinity_np(), pthread_attr_getaffinity_np(), pthread_setaffinity_np(), and pthread_getaffinity_np().  No implementation is yet in place.