Move the rdomain from struct ifnet into struct if_data. This way it

Move the rdomain from struct ifnet into struct if_data.  This way it
will be exported to userland with the existing sysctl, getifaddrs()
and routing socket (if_msghdr.ifm_data) interfaces that expose
if_data.  All programs and daemons - Apps - that call the
SIOCGIFRDOMAIN ioctl in a getifaddrs() loop or after receiving an
interface message on the routing socket can now remove the pointless
additional ioctl.  In base, that could be: dhclient, isakmpd, dhcpd,
dhcrelay, ntpd, ospfd, ripd, ifconfig.

No ABI breakage because it uses a previously unused pad field in if_data.

OK mpi@ deraadt@