ripngd: retrofit the 'network' command to the new northbound model

ripngd: retrofit the 'network' command to the new northbound model

The frr-ripngd YANG module models the ripngd "network" command
using two separate leaf-lists for simplicity: one leaf-list
for interfaces and another leaf-list for actual networks. In the
'cli_show' callbacks, display the "network" command for entries of
both leaf-lists.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>