zebra: mrib: static route support

zebra: mrib: static route support

With the MRIB being independent from the Unicast RIB, there's currently
now way to add static routes to the MRIB.  Address that by adding a
separate set of commands for MRIB static routes.

Combines these original patches:
- zebra: mrib: ip mroute command to add unicast route to MRIB for multicast RPF.
- zebra: mrib: no ip mroute: Fix removal of static multicast RPF route.
- zebra: mrib: remove unused static_add/delete_ipv4
- zebra: Cleanups to zebra_rib.
- pimd: Merge pim-only branch.

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