Let callers set a gateway and a priority to IPv6 routes. This is
necessary for implementing the RADIUS Framed-IPv6-Route attribute.

Also let ip6route_del() configure .rtm_protocol. This is already
implemented in ip6route_add(), so we need to add the ip6route_del()
counterpart. Otherwise, we couldn't delete routes that were added using
a non-zero protocol.
Signed-off-by: NGuillaume Nault <g.nault@alphalink.fr>