qdisc: expose traffic shaping info via netcfg and LinkDevice trait

LinkDevice gains qdisc_info() → String defaulting to 'none'.
EthernetLink reports current qdisc configuration:
  none  /  token_bucket rate=N burst=N tokens=N  /  priority_queue len=N max=N

TokenBucket and PriorityQueue gain public accessor methods:
  rate(), burst(), tokens() / max_len()

netcfg exposes at /scheme/netcfg/ifaces/eth0/qdisc:
  cat /scheme/netcfg/ifaces/eth0/qdisc  →  'none' (default)

Enables monitoring tools to discover current traffic shaping.
Future: wo node for configuring qdisc type + parameters.

netstack/src/link/mod.rs

@@ -49,6 +49,10 @@ pub trait LinkDevice {
         String::new()
     }
 
+    fn qdisc_info(&self) -> String {
+        "none\n".to_string()
+    }
+
     fn mtu(&self) -> usize {
         1500
     }