[BusyBox] new cp and mv

[Larry Doolittle]

So "mv file1 file2" will first attempt to copy file1 to file2,
then delete file1?  And that process will fail if there isn't
space for two copies of file1 on the disk?  And if file1 is,
for instance, a log file that a writer still holds open, that
file descriptor does not point to file2 when the mv operation
completes?

Yeech.

It looks like the old cp_mv worked [sic] like this too.

         - Larry