Part 2 fix for:
https://bugzilla.redhat.com/show_bug.cgi?id=1842440
Actually this fix on its own should be sufficient, but probably we
want the nbdkit fixes too.
Note this uses actual OCaml 4.05 features! ("let open" and the
Unix.tcgetattr functions). I checked that both features are available
on RHEL 7's OCaml.
Rich.