yang, ripngd: add 'frr-ripngd.yang' and associated stub callbacks

yang, ripngd: add 'frr-ripngd.yang' and associated stub callbacks

Introduce frr-ripngd.yang, which defines a model for managing the
FRR ripngd daemon.

Update the 'frr_yang_module_info' array of ripngd with the new
'frr-ripngd' module.

Add two new files (ripng_cli.[ch]) which should contain all ripngd
commands converted to the new northbound model. Centralizing all
commands in a single place will facilitate the process of moving
the CLI to a separate program in the future.

Add automatically generated stub callbacks in
ripng_northbound.c. These callbacks will be implemented gradually
in the following commits.

Add the confd.frr-ripngd.yang YANG module with annotations specific
to the ConfD daemon.

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