libcontainer: Don't use UsetCloseOnExec, it is racy

We can't keep file descriptors without close-on-exec except with
syscall.ForkLock held, as otherwise they could leak by accident into
other children from forks in other threads.

Instead we just use Cmd.ExtraFiles which handles all this for us.

This fixes https://github.com/dotcloud/docker/issues/4493

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Merge pull request #4498 from alexlarsson/fix-cloexec-race

libcontainer: Don't use UsetCloseOnExec, it is racy