cp: implement --sparse option

[jfinkels]

The --sparse option to cp is not implemented yet. This is an option that controls whether an optimization is applied in storing the file on the filesystem, so if we just want to get things running, it probably suffices for us to just add this argument and ignore it.

GNU cp:

$ touch a && cp --sparse=always a b
# no output

uutills cp

$ touch a && ./target/release/cp --sparse=always a b
./target/release/cp: Option 'sparse' not yet implemented.

[sylvestre]

I think it is now done