show: Add 'hinting' ability

The crtools produces not only dump in its internal format (i.e. -- pb),
but also may call other tools to create files with some state (e.g.
ip tool dumps route and ifaddr info, tmpfs will be tar-ed, iptables
utility will produce the iptables-save file, etc.).

The show functionality cannot read these images, but we can guess what
can be in the file provided by its magic.

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