Get Route by Destination Address

[ryansmick]

Get a route by its destination address. This PR updates the RouteGetRequest to add a generic type for the IP address, and adds two new functions to accept IPv4 and IPv6 addresses for the destination address. It also updates the RouteHandle's get function to remove the IP family, as it's now specified by the v4 and v6 functions on the RouteGetRequest. The implemented functionality matches the functionality of the ip route get cli command.

[cathay4t]

This changes too many public API while you can avoid it by using

    pub fn destination_address(&mut self) -> &mut self {
        self.destination_address = Some(destination_address);
        let octets = destination_address.octets().to_vec();
        self.message.header.destination_prefix_length =
            (octets.len() * 8) as u8;
        self.message.nlas.push(Nla::Destination(octets));

        self
    }

When patching a library, we should try our best to maintain API stability.

[cathay4t]

Meanwhile, have you test your patch? My old code indicate NETLINK_GET_STRICT_CHK via libc::setsockopt() is required to enable kernel space filtering.

[cathay4t]

It has been a while with a lot code changed in this crate. Please rebase your code and open new pull request if you still want to work on this.