ripngd: remove the ripng global variable

ripngd: remove the ripng global variable

This is the last step to make ripngd ready for multi-instance
support.

Remove the ripng global variable and add a "ripng" parameter
to all functions that need to know the RIPng instance they are
working on. On some functions, retrieve the RIPng instance from
the interface variable when it exists (this assumes interfaces can
pertain to one RIPng instance at most, which is ok for VRF support).

In preparation for the next commits (VRF support), add a "vrd_id"
member to the ripng structure, and use ripng->vrf_id instead of
VRF_DEFAULT wherever possible.

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