image: Introduce collect-nofree flag

Current collect helper frees the pb entry if there's
zero priv_size on cinfo. For files we'll have zero
priv_size (as entries will be collected by sub-cinfos),
while the entry in question should NOT be freed.

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