Fix disk related operations

[pawamoy]

I made a mistake. All disk operations are assuming the files are on the current filesystem. They will not work for remote systems. I'm not sure how to make it possible for remote as well.

I guess the best thing to do right now is to abort disk operation when the remote is not localhost and print a warning.

Secondly, a workaround to run disk operations on remote is to use them in callbacks on the remote server, with the aria2p listen -c /path/to/callbacks.py functionality. Run the aria2p listen command in a background process (&, maybe followed by disown, or with nohup).

Eventually, maybe we could implement a way to communicate between different aria2p instances, to be able to run disk operations on remotes.