fix return value of pthread_getaffinity_np and pthread_setaffinity_np

these functions are expected to return an error code rather than
setting errno and returning -1.

fix uninitialized output from sched_getaffinity

the sched_getaffinity syscall only fills a cpu set up to the set size
used/supported by the kernel. the rest is left untouched and userspace
is responsible for zero-filling it based on the return value of the
syscall.

add pthread_setaffinity_np and pthread_getaffinity_np functions