ripd: retrofit the 'router rip' command to the new northbound model

ripd: retrofit the 'router rip' command to the new northbound model

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

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