libcriu: Ability to fetch arguments from notifications

After a bit more thinking I found a way to fetch arguments
from notifications -- pass opaque value into callback and
provide a set of calls for exploring one.

With this we can

a) provide more data if service supplies additional fields
   in the future
b) not check the action name to decide whether or not the
   requested argument is available

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>