mv: use reflink=auto mode by default

On some filesystems (btrfs), moving a file within the filesystem may
cross subvolume boundaries and we can use the lightweight reflink copy,
similar to what cp(1) does. This is typically faster than the full file
copy. It's turned on by default because it's only an optimization during
the fall back copy and does not break user expectations or usability.

* src/mv.c (main): Set the reflink mode to AUTO.