bug#23904: Btrfs clone support in copy operations

[Eli Zaretskii]

> From: Paul Eggert <eggert@cs.ucla.edu>
> Date: Wed, 6 Jul 2016 13:48:26 +0200
> Cc: sbaugh@catern.com, Kieran Colford <kieran@kcolford.com>
> 
> Revised patch attached. Basically untested, since I don't use any file 
> systems that support FIOCLONERANGE. The patch still has FIXMEs for what 
> happens if the FIOCLONERANGE ioctl is interrupted by a signal, and for a 
> good clone chunk size (the patch guesses 1 GiB).

Thanks.

However, I wonder: are we sure users will always want this
unconditionally, completely out of their control?  Does 'cp' from
Coreutils do the same unconditionally, for example?