Pull post-4.4BSD change to sys/net/route.c from BSD/OS 4.2.

Pull post-4.4BSD change to sys/net/route.c from BSD/OS 4.2.

Have sys/net/route.c:rtrequest1(), which takes ``rt_addrinfo *''
as the argument.  Pass rt_addrinfo all the way down to rtrequest1
and ifa->ifa_rtrequest.  3rd argument of ifa->ifa_rtrequest is now
``rt_addrinfo *'' instead of ``sockaddr *'' (almost noone is
using it anyways).

Benefit: the following command now works.  Previously we needed
two route(8) invocations, "add" then "change".
# route add -inet6 default ::1 -ifp gif0

Remove unsafe typecast in rtrequest(), from ``rtentry *'' to
``sockaddr *''.  It was introduced by 4.3BSD-Reno and never
corrected.

Obtained from:	BSD/OS, NetBSD
MFC after:	1 month
PR:		kern/28360