crit: Decode some numbers into strings

There are several places in image files, where we store
integers, but these numbers actually mean some string.
E.g. socket families, states and types and tasks states.

So here's the (criu).dict option for such fields that
helps to convert the numbers into strings and back.

Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>