Routing v4 over an v6 nexthop is already well supported within zebra
(and FRR). This adds support to staticd, allowing an IPv6 nexthop to
be provided to ip route statements. For this the commands are
extended and the address family is parsed from the parameter.
When receiving nht updates from zebra, both AFIs are checked because
prefixes could exist in both. Additionally when route_node is known,
family of prefix is used instead of nexthop.
Signed-off-by: Christopher Dziomba <christopher.dziomba@telekom.de>