zebra: make MRIB lookup behaviour switchable

zebra: make MRIB lookup behaviour switchable

depending on the usage scenario (and availability of multitopology IGP
protocols, which is currently zero in Quagga), different approaches of
Multicast RPF lookups are useful.

Reference behaviours from commercial vendors are urib-only/mrib-only
(Juniper, depending on inet.2 availability) and lowest-distance (Cisco).
As we are currently without MT IGP support, mrib-first seems the most
useful default for Quagga.

Cc: Everton Marques <everton.marques@gmail.com>
Cc: Balaji G <balajig81@gmail.com>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>