Fork stdlibs os/exec package
Fork stdlibs os/exec package

It was copied from my local go version at: go version go1.9.2 linux/amd64

This is so that we can export a helper to close IO explicitly when we
need it. For more info, see this issue:
https://github.com/golang/go/issues/18874

pachyderm/pfs,pachyderm/pfs,pachyderm/pfs