Add ability to forward IPv4 packets trough IPv6 only router

[ae]

Summary

This patch add basic support to add and use IPv4 routes with IPv6 next hop.
Now it is possible to add IPv4 route entry with IPv6 next hop like this:

# route add 192.168.0.0/24 -gateway6 fe80::1%re0
add net 192.168.0.0: gateway fe80::1%re0
# netstat -rnf inet | grep 192.168
192.168.0.0/24     fe80::1%re0        UGS         re0

And when layer2 code is trying to resolve L2 address it will use NDP.
There are some pieces of code that needs to be fixed, e.g. firewall based forwarding.

[melifaro]

A slightly different version landed in D30398.

[ae]

Superseded by D30398.