libcriu: add ability to use local options structure

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>