Move the RouteAddRequest method into a separate builder

For both the `RouteAddRequest` and `RouteDelRequest`, we need to build
a route message. However only the former provides helper methods to do
so.

Instead of duplicating code, create a separate builder, and use it for
both request types.

examples/add_route.rs

@@ -1,9 +1,9 @@
 // SPDX-License-Identifier: MIT
 
-use std::env;
+use std::{env, net::Ipv4Addr};
 
 use ipnetwork::Ipv4Network;
-use rtnetlink::{new_connection, Error, Handle};
+use rtnetlink::{new_connection, Error, Handle, RouteMessageBuilder};
 
 const TEST_TABLE_ID: u32 = 299;
 
@@ -40,15 +40,12 @@ async fn add_route(
     gateway: &Ipv4Network,
     handle: Handle,
 ) -> Result<(), Error> {
-    let route = handle.route();
-    route
-        .add()
-        .v4()
+    let route = RouteMessageBuilder::<Ipv4Addr>::new()
         .destination_prefix(dest.ip(), dest.prefix())
         .gateway(gateway.ip())
         .table_id(TEST_TABLE_ID)
-        .execute()
-        .await?;
+        .build();
+    handle.route().add(route).execute().await?;
     Ok(())
 }
 

examples/add_route_pref_src.rs

@@ -4,7 +4,7 @@ use futures::TryStreamExt;
 use std::{env, net::Ipv4Addr};
 
 use ipnetwork::Ipv4Network;
-use rtnetlink::{new_connection, Error, Handle};
+use rtnetlink::{new_connection, Error, Handle, RouteMessageBuilder};
 
 #[tokio::main]
 async fn main() -> Result<(), ()> {
@@ -53,15 +53,12 @@ async fn add_route(
         .header
         .index;
 
-    let route = handle.route();
-    route
-        .add()
-        .v4()
+    let route = RouteMessageBuilder::<Ipv4Addr>::new()
         .destination_prefix(dest.ip(), dest.prefix())
         .output_interface(iface_idx)
         .pref_source(source)
-        .execute()
-        .await?;
+        .build();
+    handle.route().add(route).execute().await?;
     Ok(())
 }
 

examples/listen.rs

@@ -6,7 +6,10 @@
 use futures::stream::StreamExt;
 use netlink_sys::{AsyncSocket, SocketAddr};
 use rtnetlink::{
-    constants::{RTMGRP_IPV4_ROUTE, RTMGRP_IPV6_ROUTE},
+    constants::{
+        RTMGRP_IPV4_IFADDR, RTMGRP_IPV4_ROUTE, RTMGRP_IPV6_IFADDR,
+        RTMGRP_IPV6_ROUTE, RTMGRP_LINK,
+    },
     new_connection,
 };
 
@@ -18,7 +21,11 @@ async fn main() -> Result<(), String> {
 
     // These flags specify what kinds of broadcast messages we want to listen
     // for.
-    let mgroup_flags = RTMGRP_IPV4_ROUTE | RTMGRP_IPV6_ROUTE;
+    let mgroup_flags = RTMGRP_LINK
+        | RTMGRP_IPV4_IFADDR
+        | RTMGRP_IPV4_ROUTE
+        | RTMGRP_IPV6_IFADDR
+        | RTMGRP_IPV6_ROUTE;
 
     // A netlink socket address is created with said flags.
     let addr = SocketAddr::new(0, mgroup_flags);