zebra: support FIB override routes

zebra: support FIB override routes

FIB override routes are for routing protocols that establish
shortcut routes, or establish point-to-point routes that should
not be redistributed. Namely this is useful NHRP daemon to come.

Zebra is extended to select two entries from RIB the "best" entry
from routing protocols, and the FIB entry to install to kernel.
FIB override routes are never selected as best entry, and thus
are never adverticed to other routing daemons. The best FIB
override, or if it does not exist the otherwise best RIB is
selected as FIB entry to be installed.

Signed-off-by: Timo Teräs <timo.teras@iki.fi>
Acked-by: Donald Sharp <sharpd@cumulusnetworks.com>
[CF: Massage to fit cumulus tree]
Signed-off-by: Christian Franke <chris@opensourcerouting.org>