[3.6] bpo-33871: Fix os.sendfile(), os.writev(), os.readv(), etc. (GH-7931) (GH-8584)

[3.6] bpo-33871: Fix os.sendfile(), os.writev(), os.readv(), etc. (GH-7931) (GH-8584)

* Fix integer overflow in os.readv(), os.writev() 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.
(cherry picked from commit 9d5727326af53ddd91016d98e16ae7cf829caa95)