ripngd: retrofit the 'router ripng' command to the new northbound model

ripngd: retrofit the 'router ripng' command to the new northbound model

* Implement the northbound callbacks associated to the
  '/frr-ripngd:ripngd/instance' YANG path (the code is mostly a copy
  and paste from the original "router ripng" DEFUNs);
* Move ripng_make_socket() out of ripng_create() since creating a
  socket is an error-prone operation and thus needs to be performed
  separately during the NB_EV_PREPARE phase;
* On ripng_create(), fetch the defaults from the frr-ripngd YANG
  model;
* Convert the "[no] router ripng" CLI commands to be dumb wrappers
  around the northbound callbacks;
* On ripng_config_write(), write logic to call all 'cli_show'
  northbound callbacks defined under the '/frr-ripngd:ripngd/instance'
  YANG path.

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