babeld: Don't use an ifindex when installing IPv4 routes.

babeld: Don't use an ifindex when installing IPv4 routes.

Stand-alone babeld installs routes using both a next-hop gateway
and an interface index.  Unfortunately, this doesn't work for IPv4
under Quagga.

We now ignore the ifindex when installing IPv4 routes, which makes
Babel work for IPv4 in prefix-based networks.  Of course this breaks
IPv4 mesh networks, unless you play some tricks with your interfaces'
netmasks.