crit: Show no payload for image objects

In some images there can be quite a long "payload" -- some
raw data that is represented by base64 encoding. If we want
to explore huge images reading tons of base64 symbols can
be quite time consuming :) E.g. I a 1.5 gigs image with sysv
shmem was sent to me some time ago for investigation %)

So here is the --nopl argument for show action (decode should
produce encode-able image, so payload there is needed) that
just shows the amount of bytes in payload (if any).

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