os/exec: Failure in Cmd.Start() can orphan descriptors

by **briandellisanti**:

<pre>Inspection of os/exec.Cmd.Start() shows that it will orphan descriptors created
internally if it fails to complete successfully. Finalization will close the descriptors
eventually, but it would be better if they were closed immediately.

Specifically, if os.StartProcess() or the setupFd methods return errors, Start() bails
out immediately.</pre>
