copy: attempt copy offload with sparse files by default

copy: attempt copy offload with sparse files by default

This was seen to vastly improve performance
on NFS 4.2 systems by allowing server side copies,
with partially sparse files (avidemux generated mp4 files).

* src/copy.c (lseek_copy): Also set hole_size to 0,
i.e. enable copy_file_range(), with --sparse=auto (the default),
to enable copy offload in this case, as we've strong signal
from SEEK_DATA that we're operating on actual data and not holes here.
* NEWS: Mention the improvement.
Fixes https://bugs.gnu.org/60416