Enable renaming paths to final paths on different partition

[samuell]

Now, if writing to paths that are on a different partition, for example /tmp/foo, if / is on a different hard drive partition than your /home/ folder where you might execute the workflow, the os.Rename() call in FinalizePaths() will fail with "invalid cross-device link".

To come around this, we could check for that specific error and if so, do a proper copy and remove instead.

Some links with pointers and/or ideas:

• gist:MoveFile.go