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_default.go

@@ -0,0 +1,15 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !linux
+
+package ktimeout
+
+import (
+	"time"
+)
+
+// SetUserTimeout is a no-op on this platform.
+func SetUserTimeout(fd uintptr, timeout time.Duration) error {
+	return nil
+}