ripngd: split northbound callbacks into multiple files

ripngd: split northbound callbacks into multiple files

Rearrange the ripngd northbound callbacks as following:
* ripng_nb.h: prototypes of all northbound callbacks.
* ripng_nb.c: definition of all northbound callbacks and their
  associated YANG data paths.
* ripng_nb_config.c: implementation of YANG configuration nodes.
* ripng_nb_state.c: implementation of YANG state nodes.
* ripng_nb_rpcs.c: implementation of YANG RPCs.

This should help to keep to code more organized and easier to
maintain.

No behavior changes intended.

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