route: add /scheme/netcfg/route/gateway read-only node

Reads the default gateway from the route table:
  cat /scheme/netcfg/route/gateway
  →  'default via 192.168.1.1\n'
or 'no default route\n' if no default gateway configured.

Looks up the route for 0.0.0.0 (the default route CIDR) and
returns its 'via' address. Mirrors Linux 'ip route show default'.

netstack/src/scheme/netcfg/mod.rs

@@ -266,6 +266,17 @@ fn mk_root_node(
                     format!("routes: {}\n", count)
                 }
             },
+            "gateway" => {
+                ro [route_table] || {
+                    let table = route_table.borrow();
+                    let zero: IpAddress = IpAddress::v4(0, 0, 0, 0);
+                    let gw = table.lookup_gateway(&zero);
+                    match gw {
+                        Some(addr) => format!("default via {}\n", addr),
+                        None => "no default route\n".to_string(),
+                    }
+                }
+            },
             "add" => {
                 wo [iface, notifier, route_table] (Option<Rule>, None)
                 |cur_value, line| {