Copy offloading in IO.copy_stream
In linux 4.5, the copy offloading feature with copy_file_range(2) was introduced.
This patch enables IO.copy_stream to use it.
If "offload" keyword argument is specified, IO.copy_stream will try copy offloading.
IO.copy_stream("src", "dst", offload: true)
If copy offloading is not available on the system, "offload" option will be ignored.
When "src" and "dst" are not in the same filesystem, copy_file_range(2) must fail with EXDEV.
In this case, IO.copy_steram will fallback to another method (sendfile(2) or read/write) silently.
It depends on the filesystem what offloading techniques will be used.
Copy offloading is optional in IO.copy_stream because some techniques may change the current behavior.