net/ktimeout: add a package to set TCP user timeout

Setting a user timeout will be a more practical tuning knob for a number
of endpoints, this provides a way to set it.

Updates tailscale/corp#17587

Signed-off-by: James Tucker <james@tailscale.com>

net/ktimeout/ktimeout_test.go

@@ -0,0 +1,24 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package ktimeout
+
+import (
+	"context"
+	"fmt"
+	"net"
+	"time"
+)
+
+func ExampleUserTimeout() {
+	lc := net.ListenConfig{
+		Control: UserTimeout(30 * time.Second),
+	}
+	l, err := lc.Listen(context.TODO(), "tcp", "127.0.0.1:0")
+	if err != nil {
+		fmt.Printf("error: %v", err)
+		return
+	}
+	l.Close()
+	// Output:
+}