libnative: process spawning should not close inherited file descriptors

* The caller should be responsible for cleaning up file descriptors
* If a caller safely creates a file descriptor (via
  native::io::file::open) the returned structure (FileDesc) will try to
  clean up the file, failing in the process and writing error messages
  to the screen.
* This should not happen as the caller has no public interface for
  telling the FileDesc structure to NOT free the underlying fd.
* Alternatively, if another file is opened under the same fd held by
  the FileDesc structure returned by native::io::file::open, it will
  close the wrong file upon destruction.
auto merge of #16421 : ipetkov/rust/cmd-fd-fix-retry, r=alexcrichton

Retry pull requesting of https://github.com/rust-lang/rust/pull/16407 after accidentally messing up rebasing of branches and making bors think the PR was merged