[Rd] Redirect system2 stdout to a file on windows

[Jeroen Ooms]

According to the manual, the `stdout` argument of the `system2`
function can redirect output to a file. This seems to work on unix,
however I can't get it to work on windows. The toy example below, no
`out.txt` or `err.txt` files are created. I tried sending it to an
existing file, or expand the full file path, but with no success:

    setwd(tempdir())
    system2("whoami", stdout="out.txt", stderr="err.txt")
    file.exists("out.txt")

Am I doing something wrong or is this not supported on windows?