file_util::copy_stream support for copy_file_range on Linux 4.5

[lilianmoraru]

I was reading yesterday about copy_file_range API extension in Linux 4.5 and first thing I remembered was the stack overflow issue with the big buffer for file copying.

This is a feature request to support copy_file_range on Linux 4.5.

[tailhook]

Yeah. I'm dreaming about that too. Unfortunately, this only works in the same mountpoint. So we need to make some changes to the layout of the mountpoints in "capsule" (our temporary file system). And we'll be doing that shortly after 0.7.0 released.