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

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