ripd: remove the rip global variable

ripd: remove the rip global variable

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

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

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

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