Having ability to have your own options structure is quite nice
and allows much more flexible use of libcriu in cases when you
want to have a bunch of instances of options structures.

This patch also allows users to use raw CriuOpts structure
modified in any suitable way, whether by libcriu's criu_local_set
methods or by using protobuf-c directly.

It is also worth noting, that backward-compatibility in API and ABI
is preserved.
Signed-off-by: Ruslan Kuprieiev <rkuprieiev@cloudlinux.com>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>