* Fix integer overflow in os.readv(), os.writev(), os.preadv()
and os.pwritev() and in os.sendfile() with headers or trailers
arguments (on BSD-based OSes and MacOS).
* Fix sending the part of the file in os.sendfile() on MacOS.
Using the trailers argument could cause sending more bytes from
the input file than was specified.
Thanks Ned Deily for testing on 32-bit MacOS.