Implements streamign exec handling in both executors (i.e. universal and
libcontainer).

For creation of TTY, some incidental complexity leaked in.  The universal
executor uses github.com/kr/pty for creation of TTYs.

On the other hand, libcontainer expects a console socket and for libcontainer to
create the underlying console object on process start.  The caller can then use
`libcontainer.utils.RecvFd()` to get tty master end.

I chose github.com/kr/pty for managing TTYs here.  I tried
`github.com/containerd/console` package (which is already imported), but the
package did not work as expected on macOS.