Make macos/ios version of accept4 function public
Implement scatter/gather IO: writev & readv.

Also added tests to check them.
Update static->const
Merge pull request #5 from ok32/bug/macos-accept4

Make macos/ios version of accept4 function public
Merge pull request #8 from little-arhat/feature-scatter-gather

Implement scatter/gather IO: writev & readv.