Merge struct route and struct route_in6.

Merge struct route and struct route_in6.

Use a common struct route for both inet and inet6.  Unfortunately
struct sockaddr is shorter than sockaddr_in6, so netinet/in.h has
to be exposed from net/route.h.  Struct route has to be bsd visible
for userland as netstat kvm code inspects inp_route.  Internet PCB
and TCP SYN cache can use a plain struct route now.  All specific
sockaddr types for inet and inet6 are embeded there.

OK claudio@