zebra: add ZEBRA_IPV4_NEXTHOP_LOOKUP_MRIB

zebra: add ZEBRA_IPV4_NEXTHOP_LOOKUP_MRIB

This adds a new zapi call "ZEBRA_IPV4_NEXTHOP_LOOKUP_MRIB" performing a
Multicast RPF lookup for a given source.  Details of the lookup
behaviour are left to the zebra side of things.

Note: this is non-reactive, as in, only delivers a snapshot of the state
at a particular point in time.  There's no push notification of changes
happening to the RIB.

This combines the following 3 original patches:
- zebra: add zsend_ipv4_nexthop_lookup_mrib()
- zserv: Query mrib (SAFI_MULTICAST).
- zebra: Cleanups to zebra_rib.

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