restore: networking stack files from reflog (Phases 1-6)

Recovered from reflog commits 1c80937e and d0ecc067 after force-push data loss.
Includes: filter/, icmp_error.rs, slaac.rs, bond.rs, bridge.rs, gre.rs, ipip.rs,
qdisc.rs, tun.rs, vlan.rs, vxlan.rs, netfilter.rs, tun.rs, conntrack.rs, nat.rs,
rule.rs, table.rs, redbear-ufw/, dhcpv6d/, netdiag/ — 39 files total.

netstack/src/link/mod.rs

@@ -1,6 +1,13 @@
+pub mod bond;
+pub mod bridge;
 pub mod ethernet;
+pub mod gre;
+pub mod ipip;
 pub mod loopback;
-pub mod stp;
+pub mod qdisc;
+pub mod tun;
+pub mod vlan;
+pub mod vxlan;
 
 use std::rc::Rc;
 
@@ -30,6 +37,32 @@ pub trait LinkDevice {
 
     fn ip_address(&self) -> Option<IpCidr>;
     fn set_ip_address(&mut self, addr: IpCidr);
+
+    fn arp_table(&self) -> String {
+        String::new()
+    }
+
+    fn flush_arp(&mut self) {}
+
+    fn mtu(&self) -> usize {
+        1500
+    }
+
+    fn link_state(&self) -> &'static str {
+        "unknown"
+    }
+
+    fn statistics(&self) -> Stats {
+        Stats::default()
+    }
+}
+
+#[derive(Debug, Default, Clone, Copy)]
+pub struct Stats {
+    pub rx_bytes: u64,
+    pub rx_packets: u64,
+    pub tx_bytes: u64,
+    pub tx_packets: u64,
 }
 
 #[derive(Default)]