net/udprelay: bind multiple sockets per af on Linux

This commit uses SO_REUSEPORT (when supported) to bind multiple sockets
per address family. Increasing the number of sockets can increase
aggregate throughput when serving many peer relay client flows.
Benchmarks show 3x improvement in max aggregate bitrate in some
environments.

Updates tailscale/corp#34745

Signed-off-by: Jordan Whited <jordan@tailscale.com>

net/udprelay/server_linux.go

@@ -0,0 +1,35 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build linux
+
+package udprelay
+
+import (
+	"net"
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
+
+func listenControl(_ string, _ string, c syscall.RawConn) error {
+	c.Control(func(fd uintptr) {
+		unix.SetsockoptInt(int(fd), unix.SOL_SOCKET, unix.SO_REUSEPORT, 1)
+	})
+	return nil
+}
+
+func isReusableSocket(uc *net.UDPConn) bool {
+	rc, err := uc.SyscallConn()
+	if err != nil {
+		return false
+	}
+	var reusable bool
+	rc.Control(func(fd uintptr) {
+		val, err := unix.GetsockoptInt(int(fd), unix.SOL_SOCKET, unix.SO_REUSEPORT)
+		if err == nil && val == 1 {
+			reusable = true
+		}
+	})
+	return reusable
+}