Avoid writing .tmp file when redirecting stdout to a file

[Mytherin]

This PR fixes a regression of #2296 found by @mskyttner where writing to stdout would fail if the result was redirected to a file, because the /dev/stdout pretends to be a file when it is redirected to a file (i.e. it sets st_mode in stat as if it is a file). For now we resolve this by checking directly if the output target is /dev/stdout as there does not seem to be a good way to resolve this using file system operations itself.

We also do some clean-up of the copy to file - as the is_file_and_exists property is not necessary anymore after binding it should not be propagated further into the plan.