Require gateways for routes to be of the same address family as the
route itself.

It fixes a bug where an IPv4 route for example has an IPv6 gateway
specified:

route add 10.1.1.1 -inet6 fe80::1%fxp0

Destination  Gateway       Flags  Refs  Use  Netif Expire
10.1.1.1     fe80::1%fxp0  UGHS   0     0    fxp0

The fix rejects these illegal combinations:

route: writing to routing socket: Invalid argument
add host 10.1.1.1: gateway fe80::1%fxp0: Invalid argument

Reviewed by: KAME jinmei@isl.rdc.toshiba.co.jp
Reviewed by: andre (mentor)
Approved by: re
MFC after: 5