wgengine/magicsock: use AF_PACKET socket + BPF to read disco messages

This is entirely optional (i.e. failing in this code is non-fatal) and
only enabled on Linux for now. Additionally, this new behaviour can be
disabled by setting the TS_DEBUG_DISABLE_AF_PACKET environment variable.

Updates #3824
Replaces #5474

Co-authored-by: Andrew Dunham <andrew@du.nham.ca>
Signed-off-by: David Anderson <danderson@tailscale.com>

wgengine/magicsock/magicsock_default.go

@@ -0,0 +1,17 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !linux
+// +build !linux
+
+package magicsock
+
+import (
+	"errors"
+	"io"
+)
+
+func (c *Conn) listenRawDisco(family string) (io.Closer, error) {
+	return nil, errors.New("raw disco listening not supported on this OS")
+}