[BusyBox] new cp and mv

[Larry Doolittle]

> [mv] should try to rename the file first.  If this fails because the
> source and dest are on different devices, then it falls back to
> the really nasty copying stuff.

Ahh.  You're right.  I glossed over the rename() invocation,
it's so succinct compared to the failover implementation.
Maybe it would help to add a comment to call attention to the
primary attempt / failover sequence.

    - Larry