Fix minimal mv to work across fs

Fix minimal mv to work across fs

This is the use case in initramfs to keep data from /run to /root/run
before calling switch_root().

copy_file() doesn't yet catch EINTR, but that seems less of an issue
in initramfs.

copy_file() and much logic out of copy() is from Ulrich Dangel.
Thank you for the collaboration on this blocker for
http://bugs.debian.org/627808

While we are it require move to have mv at least 2 arguments passed.

Signed-off-by: Ulrich Dangel <uli@spamt.net>
Signed-off-by: maximilian attems <max@stro.at>
[bwh: This is no longer needed by initramfs-tools starting with version 0.121,
 so can be dropped after the stretch release]

Gbp-Pq: Name Fix-minimal-mv-to-work-across-fs