Now we have the crit utility to print images' contents in the human-readable
format, so show can be thrown out some time soon.

For now let's just deprecate it and leave functional only when the output
is asked into non-terminal. I.e. the plan shell "criu show -f" will not work.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>