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/Cargo.toml

@@ -30,6 +30,7 @@ features = [
     "std",
     "medium-ethernet", "medium-ip",
     "proto-ipv4",
+    "proto-ipv6",
     "socket-raw", "socket-icmp", "socket-udp", "socket-tcp", "socket-tcp-cubic",
     "iface-max-addr-count-8",
     "log"