beam: Fix double close of fds in SendUnix

Instead of calling syscall.Close() on the fds in sendUnix() we call
Close() on the *os.File in Send(). Otherwise the fd will be closed, but
the *os.File will continue to live, and when it is finalized the
fd will be closed (which by then may be reused and can be anything).

This also adds a note to Send() the the file is closed.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)