dd: Eliminate code duplication

[backwaterred]

dd contains functions dd_fileout and dd_stdout which repeat the same code. The same issue is present with duplicate write_blocks procedures. @miDeb suggested this patch, which solves this issue by creating a OutputTrait which can be implemented generically for all outputs. This seems to be a good solution, but others could be considered as well.

[backwaterred]

I've applied @miDeb's patch locally, and it's looking good. I will likely create a PR shortly.